feat: implement board tab with dialogs, virtualized card lists and refresh-after-write

- board: columns in id order, per-column sort via config, column width presets (240/320/480) and custom 160-720
- column header new-card and edit/delete menu; card menu delete and move-to-column
- card editor as one unit of work with dirty check and discard confirmation; nested tag picker writes new tags immediately
- schema constraint violations mapped to typed exceptions and shown inside the modal
- dialog service with owner stack so nested modals center on their parent and get FlowDirection
- repository: filtered GetCards, CountCards; view-model tests

YKanBan.Tests/Storage/Workspace/WorkspaceRepositoryTests.cs

@@ -1,5 +1,7 @@
 using Microsoft.Data.Sqlite;
 using YKanBan.Models;
+using YKanBan.Search;
+using YKanBan.Storage;
 using YKanBan.Storage.Workspace;
 using YKanBan.Tests.TestUtilities;
 
@@ -253,8 +255,87 @@ public class WorkspaceRepositoryTests
     public void AddTagWithDuplicateNameIsRejected()
     {
         using var fixture = new RepositoryFixture();
-        Assert.ThrowsExactly<SqliteException>(() =>
+        DuplicateNameException exception = Assert.ThrowsExactly<DuplicateNameException>(() =>
-            fixture.Repository.AddTag("bug", RgbColor.Parse("#123456"), ""));
+            fixture.Repository.AddTag("BUG", RgbColor.Parse("#123456"), ""));
+        Assert.AreEqual(NamedEntity.Tag, exception.Entity);
+        Assert.AreEqual("BUG", exception.Name);
+        Assert.AreEqual(2, fixture.Repository.GetTags().Count);
+    }
+
+    [TestMethod]
+    public void AddTagWithEmptyNameIsRejected()
+    {
+        using var fixture = new RepositoryFixture();
+        NameRequiredException exception = Assert.ThrowsExactly<NameRequiredException>(() =>
+            fixture.Repository.AddTag("", RgbColor.Parse("#123456"), ""));
+        Assert.AreEqual(NamedEntity.Tag, exception.Entity);
+    }
+
+    [TestMethod]
+    public void UpdateTagToExistingNameIsRejectedAndKeepsTheRow()
+    {
+        using var fixture = new RepositoryFixture();
+        Assert.ThrowsExactly<DuplicateNameException>(() =>
+            fixture.Repository.UpdateTag(fixture.UiTagId, "Bug", RgbColor.Parse("#00ff00"), ""));
+        Assert.AreEqual("ui", fixture.Repository.GetTags().Single(tag => tag.Id == fixture.UiTagId).Name);
+    }
+
+    [TestMethod]
+    public void AddColumnWithDuplicateTitleIsRejectedCaseInsensitively()
+    {
+        using var fixture = new RepositoryFixture();
+        DuplicateNameException exception = Assert.ThrowsExactly<DuplicateNameException>(() =>
+            fixture.Repository.AddColumn("COL", ""));
+        Assert.AreEqual(NamedEntity.Column, exception.Entity);
+        Assert.AreEqual(1, fixture.Repository.GetColumns().Count);
+    }
+
+    [TestMethod]
+    public void AddColumnWithEmptyTitleIsRejected()
+    {
+        using var fixture = new RepositoryFixture();
+        NameRequiredException exception = Assert.ThrowsExactly<NameRequiredException>(() =>
+            fixture.Repository.AddColumn("", "description"));
+        Assert.AreEqual(NamedEntity.Column, exception.Entity);
+        Assert.AreEqual(1, fixture.Repository.GetColumns().Count);
+    }
+
+    [TestMethod]
+    public void UpdateColumnConstraintViolationsAreTyped()
+    {
+        using var fixture = new RepositoryFixture();
+        ColumnModel other = fixture.Repository.AddColumn("Other", "");
+
+        Assert.ThrowsExactly<DuplicateNameException>(() => fixture.Repository.UpdateColumn(other.Id, "col", ""));
+        Assert.ThrowsExactly<NameRequiredException>(() => fixture.Repository.UpdateColumn(other.Id, "", ""));
+        Assert.AreEqual("Other", fixture.Repository.GetColumn(other.Id)!.Title);
+    }
+
+    [TestMethod]
+    public void GetCardsWithFilterReturnsMatchesWithAllOfTheirTags()
+    {
+        using var fixture = new RepositoryFixture();
+        fixture.Repository.AddCard(fixture.ColumnId, "Other", "", []);
+
+        // Filtering by one tag must still load both tags of the matching card.
+        CompiledSearch filter = SearchSqlCompiler.Compile(SearchQueryParser.Parse("tag:ui")!);
+        IReadOnlyList<CardModel> cards = fixture.Repository.GetCards(filter);
+
+        Assert.AreEqual(1, cards.Count);
+        Assert.AreEqual(fixture.CardId, cards[0].Id);
+        CollectionAssert.AreEqual(new[] { "bug", "ui" }, cards[0].Tags.Select(tag => tag.Name).ToArray());
+        Assert.AreEqual(2, fixture.Repository.GetCards(null).Count);
+    }
+
+    [TestMethod]
+    public void CountCardsCountsPerColumn()
+    {
+        using var fixture = new RepositoryFixture();
+        ColumnModel empty = fixture.Repository.AddColumn("Empty", "");
+        fixture.Repository.AddCard(fixture.ColumnId, "", "", []);
+
+        Assert.AreEqual(2L, fixture.Repository.CountCards(fixture.ColumnId));
+        Assert.AreEqual(0L, fixture.Repository.CountCards(empty.Id));
     }
 
     [TestMethod]

YKanBan.Tests/ViewModels/BoardViewModelTests.cs

@@ -0,0 +1,393 @@
+using YKanBan.Models;
+using YKanBan.Search;
+using YKanBan.Storage;
+using YKanBan.Storage.AppData;
+using YKanBan.Storage.Workspace;
+using YKanBan.Tests.TestUtilities;
+using YKanBan.ViewModels.Board;
+using YKanBan.ViewModels.Dialogs;
+
+namespace YKanBan.Tests.ViewModels;
+
+/// <summary>
+/// View-model tests of the board and its dialogs against a real temporary
+/// workspace, with a scripted dialog service standing in for the modal windows.
+/// </summary>
+[TestClass]
+public class BoardViewModelTests
+{
+    /// <summary>
+    /// Records every dialog shown and lets a test script answer it. The result
+    /// is whatever the dialog closed with; a dialog the script leaves open
+    /// counts as dismissed.
+    /// </summary>
+    private sealed class ScriptedDialogService : IDialogService
+    {
+        public List<DialogViewModelBase> Shown { get; } = [];
+
+        public Func<DialogViewModelBase, Task> Script { get; set; } = _ => Task.CompletedTask;
+
+        public async Task<bool> ShowAsync(DialogViewModelBase dialog)
+        {
+            Shown.Add(dialog);
+            bool? result = null;
+            dialog.CloseRequested += (_, closed) => result ??= closed;
+            await Script(dialog);
+            return result ?? false;
+        }
+    }
+
+    /// <summary>Answers every confirmation dialog with the given choice.</summary>
+    private static Func<DialogViewModelBase, Task> AnswerConfirmations(bool confirm) => dialog =>
+    {
+        if (dialog is ConfirmDialogViewModel confirmation)
+        {
+            if (confirm)
+            {
+                confirmation.ConfirmCommand.Execute(null);
+                return Task.CompletedTask;
+            }
+            return confirmation.CancelCommand.ExecuteAsync(null);
+        }
+        return Task.CompletedTask;
+    };
+
+    private sealed class Fixture : IDisposable
+    {
+        public TempDirectory Directory { get; } = new();
+
+        public WorkspaceRepository Repository { get; }
+
+        public AppConfig Config { get; } = new();
+
+        public ScriptedDialogService Dialogs { get; } = new();
+
+        public Fixture()
+        {
+            WorkspaceInitializer.Initialize(Directory.FullPath);
+            Repository = new WorkspaceRepository(Directory.FullPath);
+        }
+
+        public BoardViewModel CreateBoard() => new(Repository, Config, Dialogs);
+
+        public void Dispose()
+        {
+            Repository.Dispose();
+            Directory.Dispose();
+        }
+    }
+
+    [TestMethod]
+    public void RefreshListsColumnsInIdOrderAndSortsCardsWithinEachColumn()
+    {
+        using var fixture = new Fixture();
+        ColumnModel first = fixture.Repository.AddColumn("A", "");
+        ColumnModel second = fixture.Repository.AddColumn("B", "");
+        fixture.Repository.AddCard(first.Id, "zeta", "", []);
+        fixture.Repository.AddCard(first.Id, "alpha", "", []);
+        fixture.Repository.AddCard(second.Id, "x", "", []);
+
+        fixture.Config.Sort.Card = CardSortOption.Title;
+        BoardViewModel board = fixture.CreateBoard();
+
+        CollectionAssert.AreEqual(new[] { "A", "B" }, board.Columns.Select(column => column.Title).ToArray());
+        CollectionAssert.AreEqual(new[] { "alpha", "zeta" }, board.Columns[0].Cards.Select(card => card.Title).ToArray());
+        Assert.AreEqual(1, board.Columns[1].CardCount);
+    }
+
+    [TestMethod]
+    public void ChangingTheSortPersistsItToTheConfigurationAndResorts()
+    {
+        using var fixture = new Fixture();
+        ColumnModel column = fixture.Repository.AddColumn("A", "");
+        fixture.Repository.AddCard(column.Id, "b", "", []);
+        fixture.Repository.AddCard(column.Id, "a", "", []);
+        BoardViewModel board = fixture.CreateBoard();
+        CollectionAssert.AreEqual(new[] { "b", "a" }, board.Columns[0].Cards.Select(card => card.Title).ToArray());
+
+        board.SelectedSort = board.SortOptions.Single(option => (CardSortOption)option.Value == CardSortOption.Title);
+
+        Assert.AreEqual(CardSortOption.Title, fixture.Config.Sort.Card);
+        CollectionAssert.AreEqual(new[] { "a", "b" }, board.Columns[0].Cards.Select(card => card.Title).ToArray());
+    }
+
+    [TestMethod]
+    public async Task WritesRefreshWithTheActiveFilterAndKeepEmptyColumns()
+    {
+        using var fixture = new Fixture();
+        ColumnModel todo = fixture.Repository.AddColumn("Todo", "");
+        ColumnModel done = fixture.Repository.AddColumn("Done", "");
+        CardModel match = fixture.Repository.AddCard(todo.Id, "match me", "", []);
+        fixture.Repository.AddCard(todo.Id, "other", "", []);
+
+        BoardViewModel board = fixture.CreateBoard();
+        board.ActiveFilter = SearchSqlCompiler.Compile(SearchQueryParser.Parse("match")!);
+        board.Refresh();
+        Assert.AreEqual(1, board.Columns[0].CardCount);
+
+        // A move is a write: the board re-queries with the active filter.
+        CardViewModel card = board.Columns[0].Cards.Single();
+        MoveTargetViewModel target = card.MoveTargets.Single(entry => entry.ColumnId == done.Id);
+        Assert.IsFalse(card.MoveTargets.Single(entry => entry.ColumnId == todo.Id).IsEnabled);
+        await target.Command.ExecuteAsync(target.ColumnId);
+
+        Assert.AreEqual(2, board.Columns.Count, "Columns emptied by the filter stay on the board.");
+        Assert.AreEqual(0, board.Columns[0].CardCount);
+        Assert.AreEqual(match.Id, board.Columns[1].Cards.Single().Card.Id);
+    }
+
+    [TestMethod]
+    public async Task DeclinedColumnDeletionKeepsTheColumnAndShowsItsCardCount()
+    {
+        using var fixture = new Fixture();
+        ColumnModel column = fixture.Repository.AddColumn("A", "");
+        fixture.Repository.AddCard(column.Id, "", "", []);
+        fixture.Repository.AddCard(column.Id, "", "", []);
+        BoardViewModel board = fixture.CreateBoard();
+        fixture.Dialogs.Script = AnswerConfirmations(false);
+
+        await board.Columns[0].DeleteCommand.ExecuteAsync(null);
+
+        var confirmation = (ConfirmDialogViewModel)fixture.Dialogs.Shown.Single();
+        StringAssert.Contains(confirmation.Message, "2");
+        Assert.AreEqual(1, fixture.Repository.GetColumns().Count);
+    }
+
+    [TestMethod]
+    public async Task ColumnDeletionWithoutConfirmationDeletesImmediately()
+    {
+        using var fixture = new Fixture();
+        fixture.Repository.AddColumn("A", "");
+        fixture.Config.Confirmations.DeleteColumn = false;
+        BoardViewModel board = fixture.CreateBoard();
+
+        await board.Columns[0].DeleteCommand.ExecuteAsync(null);
+
+        Assert.AreEqual(0, fixture.Dialogs.Shown.Count);
+        Assert.AreEqual(0, board.Columns.Count);
+    }
+
+    [TestMethod]
+    public async Task ConfirmedCardDeletionDeletesTheCard()
+    {
+        using var fixture = new Fixture();
+        ColumnModel column = fixture.Repository.AddColumn("A", "");
+        fixture.Repository.AddCard(column.Id, "gone", "", []);
+        BoardViewModel board = fixture.CreateBoard();
+        fixture.Dialogs.Script = AnswerConfirmations(true);
+
+        await board.Columns[0].Cards[0].DeleteCommand.ExecuteAsync(null);
+
+        Assert.AreEqual(0, board.Columns[0].CardCount);
+        Assert.AreEqual(0, fixture.Repository.GetCards().Count);
+    }
+
+    [TestMethod]
+    public async Task DuplicateColumnTitleIsReportedInsideTheDialog()
+    {
+        using var fixture = new Fixture();
+        fixture.Repository.AddColumn("Todo", "");
+        BoardViewModel board = fixture.CreateBoard();
+        ColumnEditorDialogViewModel? editor = null;
+        bool closed = false;
+        fixture.Dialogs.Script = dialog =>
+        {
+            editor = (ColumnEditorDialogViewModel)dialog;
+            editor.CloseRequested += (_, _) => closed = true;
+            editor.ColumnTitle = "TODO";
+            editor.ConfirmCommand.Execute(null);
+            return Task.CompletedTask;
+        };
+
+        await board.AddColumnCommand.ExecuteAsync(null);
+
+        Assert.IsNotNull(editor);
+        Assert.IsFalse(closed, "The dialog stays open on a constraint violation.");
+        Assert.IsTrue(editor.HasError);
+        StringAssert.Contains(editor.ErrorMessage, "TODO");
+        Assert.AreEqual(1, fixture.Repository.GetColumns().Count);
+    }
+
+    [TestMethod]
+    public async Task NewCardIsWrittenWithItsTagsOnConfirmation()
+    {
+        using var fixture = new Fixture();
+        ColumnModel column = fixture.Repository.AddColumn("A", "");
+        TagModel bug = fixture.Repository.AddTag("bug", new RgbColor(255, 0, 0), "");
+        BoardViewModel board = fixture.CreateBoard();
+        fixture.Dialogs.Script = async dialog =>
+        {
+            switch (dialog)
+            {
+                case CardEditorDialogViewModel editor:
+                    editor.CardTitle = "Fix it";
+                    editor.Content = "Details";
+                    await editor.AddTagCommand.ExecuteAsync(null);
+                    editor.ConfirmCommand.Execute(null);
+                    break;
+                case TagPickerDialogViewModel picker:
+                    picker.PickCommand.Execute(picker.FilteredTags.Single());
+                    break;
+            }
+        };
+
+        await board.Columns[0].AddCardCommand.ExecuteAsync(null);
+
+        CardModel card = fixture.Repository.GetCards().Single();
+        Assert.AreEqual("Fix it", card.Title);
+        Assert.AreEqual("Details", card.Content);
+        Assert.AreEqual(bug.Id, card.Tags.Single().Id);
+        Assert.AreEqual(card.Id, board.Columns[0].Cards.Single().Card.Id);
+    }
+
+    [TestMethod]
+    public void UnchangedEditConfirmationWritesNothing()
+    {
+        using var fixture = new Fixture();
+        ColumnModel column = fixture.Repository.AddColumn("A", "");
+        CardModel card = fixture.Repository.AddCard(column.Id, "t", "c", []);
+        using (var connection = SqliteTestHelper.OpenWorkspace(fixture.Directory.FullPath))
+        {
+            SqliteTestHelper.Exec(connection, "UPDATE cards SET updated_at = 100;");
+        }
+        card = fixture.Repository.GetCard(card.Id)!;
+
+        var editor = CardEditorDialogViewModel.ForExistingCard(fixture.Repository, fixture.Dialogs, card, confirmDiscard: true);
+        editor.CardTitle = "changed";
+        editor.CardTitle = "t";
+        Assert.IsFalse(editor.IsDirty);
+        editor.ConfirmCommand.Execute(null);
+
+        Assert.AreEqual(100, fixture.Repository.GetCard(card.Id)!.UpdatedAt);
+    }
+
+    [TestMethod]
+    public async Task DirtyCancelAsksAndStaysOpenWhenDeclined()
+    {
+        using var fixture = new Fixture();
+        ColumnModel column = fixture.Repository.AddColumn("A", "");
+        var editor = CardEditorDialogViewModel.ForNewCard(fixture.Repository, fixture.Dialogs, column.Id, confirmDiscard: true);
+        bool? closedWith = null;
+        editor.CloseRequested += (_, result) => closedWith = result;
+        editor.Content = "draft";
+
+        fixture.Dialogs.Script = AnswerConfirmations(false);
+        await editor.RequestCancelAsync();
+        Assert.IsNull(closedWith, "Declining the discard keeps the editor open.");
+
+        fixture.Dialogs.Script = AnswerConfirmations(true);
+        await editor.RequestCancelAsync();
+        Assert.IsFalse(closedWith);
+        Assert.AreEqual(0, fixture.Repository.GetCards().Count);
+    }
+
+    [TestMethod]
+    public async Task CleanCancelClosesWithoutAsking()
+    {
+        using var fixture = new Fixture();
+        ColumnModel column = fixture.Repository.AddColumn("A", "");
+        var editor = CardEditorDialogViewModel.ForNewCard(fixture.Repository, fixture.Dialogs, column.Id, confirmDiscard: true);
+        bool? closedWith = null;
+        editor.CloseRequested += (_, result) => closedWith = result;
+
+        await editor.RequestCancelAsync();
+
+        Assert.IsFalse(closedWith);
+        Assert.AreEqual(0, fixture.Dialogs.Shown.Count);
+    }
+
+    [TestMethod]
+    public async Task TagCreatedInThePickerSurvivesACancelledCardEdit()
+    {
+        using var fixture = new Fixture();
+        ColumnModel column = fixture.Repository.AddColumn("A", "");
+        var editor = CardEditorDialogViewModel.ForNewCard(fixture.Repository, fixture.Dialogs, column.Id, confirmDiscard: false);
+        fixture.Dialogs.Script = dialog =>
+        {
+            if (dialog is TagPickerDialogViewModel picker)
+            {
+                picker.NewName = "fresh";
+                picker.NewColor = "#123456";
+                picker.CreateCommand.Execute(null);
+            }
+            return Task.CompletedTask;
+        };
+
+        await editor.AddTagCommand.ExecuteAsync(null);
+        Assert.AreEqual("fresh", editor.Tags.Single().Name);
+        await editor.RequestCancelAsync();
+
+        Assert.AreEqual("fresh", fixture.Repository.GetTags().Single().Name);
+        Assert.AreEqual(0, fixture.Repository.GetCards().Count);
+    }
+
+    [TestMethod]
+    public void TagPickerReportsInvalidColorAndDuplicateNameWithoutClosing()
+    {
+        using var fixture = new Fixture();
+        fixture.Repository.AddTag("bug", new RgbColor(1, 2, 3), "");
+        var picker = new TagPickerDialogViewModel(fixture.Repository, []);
+        bool closed = false;
+        picker.CloseRequested += (_, _) => closed = true;
+
+        picker.NewName = "new";
+        picker.NewColor = "red";
+        picker.CreateCommand.Execute(null);
+        Assert.AreEqual(YKanBan.Resources.Error_Tag_InvalidColor, picker.ErrorMessage);
+
+        picker.NewName = "BUG";
+        picker.NewColor = "#ff0000";
+        picker.CreateCommand.Execute(null);
+        StringAssert.Contains(picker.ErrorMessage, "BUG");
+
+        Assert.IsFalse(closed);
+        Assert.AreEqual(1, fixture.Repository.GetTags().Count);
+    }
+
+    [TestMethod]
+    public void TagPickerHidesAssignedTagsAndFiltersCaseInsensitively()
+    {
+        using var fixture = new Fixture();
+        TagModel bug = fixture.Repository.AddTag("Bug", new RgbColor(1, 2, 3), "");
+        fixture.Repository.AddTag("UI", new RgbColor(1, 2, 3), "");
+        fixture.Repository.AddTag("Build", new RgbColor(1, 2, 3), "");
+        var picker = new TagPickerDialogViewModel(fixture.Repository, [bug.Id]);
+
+        picker.FilterText = "bu";
+        CollectionAssert.AreEqual(new[] { "Build" }, picker.FilteredTags.Select(tag => tag.Name).ToArray());
+
+        picker.FilterText = "zzz";
+        Assert.IsTrue(picker.NoMatches);
+    }
+
+    [TestMethod]
+    public void ColumnWidthDialogCommitsPresetsAndClampsCustomPixels()
+    {
+        var settings = new ColumnWidthSettings();
+        var dialog = new ColumnWidthDialogViewModel(settings) { IsStandard = false, IsCustom = true, CustomPixels = 5000 };
+        dialog.ConfirmCommand.Execute(null);
+        Assert.AreEqual(ColumnWidthPreset.Custom, settings.Preset);
+        Assert.AreEqual(AppConfig.MaxColumnPixels, settings.CustomPixels);
+        Assert.AreEqual(AppConfig.MaxColumnPixels, ColumnWidths.Resolve(settings));
+
+        dialog = new ColumnWidthDialogViewModel(settings) { IsCustom = false, IsWide = true };
+        dialog.ConfirmCommand.Execute(null);
+        Assert.AreEqual(ColumnWidthPreset.Wide, settings.Preset);
+        Assert.AreEqual(ColumnWidths.Wide, ColumnWidths.Resolve(settings));
+    }
+
+    [TestMethod]
+    public void CardHeaderAndExcerptFollowTheDisplayRules()
+    {
+        var untitled = new CardModel(61, 1, "", "", 0, 0, []);
+        var titled = untitled with { Title = "Foo bar" };
+        Assert.AreEqual("#61", CardViewModel.FormatHeader(untitled));
+        Assert.AreEqual("#61 Foo bar", CardViewModel.FormatHeader(titled));
+
+        Assert.AreEqual("short", CardViewModel.MakeExcerpt("  short \n"));
+        string longContent = string.Concat(Enumerable.Repeat("😀", CardViewModel.ExcerptLength + 5));
+        string excerpt = CardViewModel.MakeExcerpt(longContent);
+        Assert.IsTrue(excerpt.EndsWith('…'));
+        Assert.AreEqual((CardViewModel.ExcerptLength * 2) + 1, excerpt.Length, "Surrogate pairs are kept whole.");
+    }
+}

YKanBan/App.axaml

@@ -26,10 +26,20 @@
                 <ResourceDictionary x:Key="Light">
                     <SolidColorBrush x:Key="TopBarBackgroundBrush" Color="#F6F7F9"/>
                     <SolidColorBrush x:Key="TopBarBorderBrush" Color="#E2E4EA"/>
+                    <SolidColorBrush x:Key="PanelBorderBrush" Color="#E2E4EA"/>
+                    <SolidColorBrush x:Key="ColumnBackgroundBrush" Color="#F3F4F7"/>
+                    <SolidColorBrush x:Key="CardBackgroundBrush" Color="#FFFFFF"/>
+                    <SolidColorBrush x:Key="CardForegroundBrush" Color="#1C1F23"/>
+                    <SolidColorBrush x:Key="ErrorTextBrush" Color="#D52515"/>
                 </ResourceDictionary>
                 <ResourceDictionary x:Key="Dark">
                     <SolidColorBrush x:Key="TopBarBackgroundBrush" Color="#1C1E24"/>
                     <SolidColorBrush x:Key="TopBarBorderBrush" Color="#2C2F37"/>
+                    <SolidColorBrush x:Key="PanelBorderBrush" Color="#33363F"/>
+                    <SolidColorBrush x:Key="ColumnBackgroundBrush" Color="#1A1C21"/>
+                    <SolidColorBrush x:Key="CardBackgroundBrush" Color="#25282F"/>
+                    <SolidColorBrush x:Key="CardForegroundBrush" Color="#E6E8EC"/>
+                    <SolidColorBrush x:Key="ErrorTextBrush" Color="#FC725A"/>
                 </ResourceDictionary>
             </ResourceDictionary.ThemeDictionaries>
         </ResourceDictionary>

YKanBan/App.axaml.cs

@@ -58,12 +58,15 @@ public partial class App : Application
                 : FlowDirection.LeftToRight;
             ApplyTheme(Services.Config.Theme);
 
-            var viewModel = new MainWindowViewModel(Services, desktop.Args ?? []);
+            var dialogs = new DialogService();
-            desktop.MainWindow = new MainWindow
+            var viewModel = new MainWindowViewModel(Services, dialogs, desktop.Args ?? []);
+            var mainWindow = new MainWindow
             {
                 DataContext = viewModel,
                 FlowDirection = UiFlowDirection,
             };
+            dialogs.Attach(mainWindow);
+            desktop.MainWindow = mainWindow;
         }
 
         base.OnFrameworkInitializationCompleted();

YKanBan/Assets/Locales/Resources.resx

@@ -184,4 +184,160 @@
   <data name="Export_TagsTableHeader" xml:space="preserve">
     <value>| Name | Color | Description | Usage |</value>
   </data>
+  <data name="Common_Delete" xml:space="preserve">
+    <value>Delete</value>
+  </data>
+  <data name="Common_Discard" xml:space="preserve">
+    <value>Discard</value>
+  </data>
+  <data name="Common_TimePattern" xml:space="preserve">
+    <value>yyyy-MM-dd HH:mm</value>
+  </data>
+  <data name="Board_AddColumn" xml:space="preserve">
+    <value>New column</value>
+  </data>
+  <data name="Board_ColumnWidth" xml:space="preserve">
+    <value>Column width</value>
+  </data>
+  <data name="Board_Sort" xml:space="preserve">
+    <value>Card order</value>
+  </data>
+  <data name="Board_AddCard" xml:space="preserve">
+    <value>New card</value>
+  </data>
+  <data name="Sort_Id" xml:space="preserve">
+    <value>By id</value>
+  </data>
+  <data name="Sort_UpdatedAt" xml:space="preserve">
+    <value>Recently modified</value>
+  </data>
+  <data name="Sort_Title" xml:space="preserve">
+    <value>By title</value>
+  </data>
+  <data name="Menu_More" xml:space="preserve">
+    <value>More</value>
+  </data>
+  <data name="Menu_Edit" xml:space="preserve">
+    <value>Edit</value>
+  </data>
+  <data name="Menu_Delete" xml:space="preserve">
+    <value>Delete</value>
+  </data>
+  <data name="Menu_MoveTo" xml:space="preserve">
+    <value>Move to column</value>
+  </data>
+  <data name="Field_Title" xml:space="preserve">
+    <value>Title</value>
+  </data>
+  <data name="Field_Description" xml:space="preserve">
+    <value>Description</value>
+  </data>
+  <data name="Field_Content" xml:space="preserve">
+    <value>Content</value>
+  </data>
+  <data name="Field_Tags" xml:space="preserve">
+    <value>Tags</value>
+  </data>
+  <data name="Field_Name" xml:space="preserve">
+    <value>Name</value>
+  </data>
+  <data name="Field_Color" xml:space="preserve">
+    <value>#RRGGBB</value>
+  </data>
+  <data name="ColumnEditor_TitleNew" xml:space="preserve">
+    <value>New column</value>
+  </data>
+  <data name="ColumnEditor_TitleEdit" xml:space="preserve">
+    <value>Edit column</value>
+  </data>
+  <data name="ColumnWidth_Title" xml:space="preserve">
+    <value>Column width</value>
+  </data>
+  <data name="ColumnWidth_Narrow" xml:space="preserve">
+    <value>Narrow ({0} px)</value>
+  </data>
+  <data name="ColumnWidth_Standard" xml:space="preserve">
+    <value>Standard ({0} px)</value>
+  </data>
+  <data name="ColumnWidth_Wide" xml:space="preserve">
+    <value>Wide ({0} px)</value>
+  </data>
+  <data name="ColumnWidth_Custom" xml:space="preserve">
+    <value>Custom</value>
+  </data>
+  <data name="ColumnWidth_Range" xml:space="preserve">
+    <value>{0}–{1} px</value>
+  </data>
+  <data name="CardEditor_TitleNew" xml:space="preserve">
+    <value>New card</value>
+  </data>
+  <data name="CardEditor_TitleEdit" xml:space="preserve">
+    <value>Edit card</value>
+  </data>
+  <data name="CardEditor_TitleWatermark" xml:space="preserve">
+    <value>No title</value>
+  </data>
+  <data name="CardEditor_Created" xml:space="preserve">
+    <value>Created {0}</value>
+  </data>
+  <data name="CardEditor_Modified" xml:space="preserve">
+    <value>Modified {0}</value>
+  </data>
+  <data name="CardEditor_AddTag" xml:space="preserve">
+    <value>Add tag</value>
+  </data>
+  <data name="CardEditor_RemoveTag" xml:space="preserve">
+    <value>Remove tag</value>
+  </data>
+  <data name="TagPicker_Title" xml:space="preserve">
+    <value>Add tag</value>
+  </data>
+  <data name="TagPicker_FilterWatermark" xml:space="preserve">
+    <value>Filter tags</value>
+  </data>
+  <data name="TagPicker_NoMatches" xml:space="preserve">
+    <value>No matching tags</value>
+  </data>
+  <data name="TagPicker_NewTag" xml:space="preserve">
+    <value>New tag</value>
+  </data>
+  <data name="TagPicker_Create" xml:space="preserve">
+    <value>Create and add</value>
+  </data>
+  <data name="Confirm_Title" xml:space="preserve">
+    <value>Confirm</value>
+  </data>
+  <data name="Confirm_DeleteCard" xml:space="preserve">
+    <value>Delete card "{0}"?</value>
+  </data>
+  <data name="Confirm_DeleteColumn" xml:space="preserve">
+    <value>Delete column "{0}" and its {1} card(s)?</value>
+  </data>
+  <data name="Confirm_DiscardEdit" xml:space="preserve">
+    <value>Discard the unsaved changes?</value>
+  </data>
+  <data name="Error_Column_TitleRequired" xml:space="preserve">
+    <value>The column title is required.</value>
+  </data>
+  <data name="Error_Column_Duplicate" xml:space="preserve">
+    <value>A column named "{0}" already exists (names are case-insensitive).</value>
+  </data>
+  <data name="Error_Tag_NameRequired" xml:space="preserve">
+    <value>The tag name is required.</value>
+  </data>
+  <data name="Error_Tag_Duplicate" xml:space="preserve">
+    <value>A tag named "{0}" already exists (names are case-insensitive).</value>
+  </data>
+  <data name="Error_Tag_InvalidColor" xml:space="preserve">
+    <value>The color must be written as #RRGGBB.</value>
+  </data>
+  <data name="Error_SaveFailed" xml:space="preserve">
+    <value>Saving failed: {0}</value>
+  </data>
+  <data name="Error_Operation_Title" xml:space="preserve">
+    <value>Operation failed</value>
+  </data>
+  <data name="Error_Operation_Message" xml:space="preserve">
+    <value>The change could not be written to the workspace. The board shows the stored state.</value>
+  </data>
 </root>

YKanBan/Assets/Locales/Resources.zh-Hans.resx

@@ -184,4 +184,160 @@
   <data name="Export_TagsTableHeader" xml:space="preserve">
     <value>| 名称 | 颜色 | 描述 | 使用数 |</value>
   </data>
+  <data name="Common_Delete" xml:space="preserve">
+    <value>删除</value>
+  </data>
+  <data name="Common_Discard" xml:space="preserve">
+    <value>丢弃</value>
+  </data>
+  <data name="Common_TimePattern" xml:space="preserve">
+    <value>yyyy年M月d日 HH:mm</value>
+  </data>
+  <data name="Board_AddColumn" xml:space="preserve">
+    <value>新建列</value>
+  </data>
+  <data name="Board_ColumnWidth" xml:space="preserve">
+    <value>列宽</value>
+  </data>
+  <data name="Board_Sort" xml:space="preserve">
+    <value>卡片排序</value>
+  </data>
+  <data name="Board_AddCard" xml:space="preserve">
+    <value>新建卡片</value>
+  </data>
+  <data name="Sort_Id" xml:space="preserve">
+    <value>按 ID</value>
+  </data>
+  <data name="Sort_UpdatedAt" xml:space="preserve">
+    <value>最近修改</value>
+  </data>
+  <data name="Sort_Title" xml:space="preserve">
+    <value>按标题</value>
+  </data>
+  <data name="Menu_More" xml:space="preserve">
+    <value>更多</value>
+  </data>
+  <data name="Menu_Edit" xml:space="preserve">
+    <value>编辑</value>
+  </data>
+  <data name="Menu_Delete" xml:space="preserve">
+    <value>删除</value>
+  </data>
+  <data name="Menu_MoveTo" xml:space="preserve">
+    <value>移动到列</value>
+  </data>
+  <data name="Field_Title" xml:space="preserve">
+    <value>标题</value>
+  </data>
+  <data name="Field_Description" xml:space="preserve">
+    <value>描述</value>
+  </data>
+  <data name="Field_Content" xml:space="preserve">
+    <value>内容</value>
+  </data>
+  <data name="Field_Tags" xml:space="preserve">
+    <value>标签</value>
+  </data>
+  <data name="Field_Name" xml:space="preserve">
+    <value>名称</value>
+  </data>
+  <data name="Field_Color" xml:space="preserve">
+    <value>#RRGGBB</value>
+  </data>
+  <data name="ColumnEditor_TitleNew" xml:space="preserve">
+    <value>新建列</value>
+  </data>
+  <data name="ColumnEditor_TitleEdit" xml:space="preserve">
+    <value>编辑列</value>
+  </data>
+  <data name="ColumnWidth_Title" xml:space="preserve">
+    <value>列宽</value>
+  </data>
+  <data name="ColumnWidth_Narrow" xml:space="preserve">
+    <value>窄（{0} px）</value>
+  </data>
+  <data name="ColumnWidth_Standard" xml:space="preserve">
+    <value>标准（{0} px）</value>
+  </data>
+  <data name="ColumnWidth_Wide" xml:space="preserve">
+    <value>宽（{0} px）</value>
+  </data>
+  <data name="ColumnWidth_Custom" xml:space="preserve">
+    <value>自定义</value>
+  </data>
+  <data name="ColumnWidth_Range" xml:space="preserve">
+    <value>{0}–{1} px</value>
+  </data>
+  <data name="CardEditor_TitleNew" xml:space="preserve">
+    <value>新建卡片</value>
+  </data>
+  <data name="CardEditor_TitleEdit" xml:space="preserve">
+    <value>编辑卡片</value>
+  </data>
+  <data name="CardEditor_TitleWatermark" xml:space="preserve">
+    <value>无标题</value>
+  </data>
+  <data name="CardEditor_Created" xml:space="preserve">
+    <value>创建于 {0}</value>
+  </data>
+  <data name="CardEditor_Modified" xml:space="preserve">
+    <value>修改于 {0}</value>
+  </data>
+  <data name="CardEditor_AddTag" xml:space="preserve">
+    <value>添加标签</value>
+  </data>
+  <data name="CardEditor_RemoveTag" xml:space="preserve">
+    <value>移除标签</value>
+  </data>
+  <data name="TagPicker_Title" xml:space="preserve">
+    <value>添加标签</value>
+  </data>
+  <data name="TagPicker_FilterWatermark" xml:space="preserve">
+    <value>筛选标签</value>
+  </data>
+  <data name="TagPicker_NoMatches" xml:space="preserve">
+    <value>没有匹配的标签</value>
+  </data>
+  <data name="TagPicker_NewTag" xml:space="preserve">
+    <value>新建标签</value>
+  </data>
+  <data name="TagPicker_Create" xml:space="preserve">
+    <value>新建并添加</value>
+  </data>
+  <data name="Confirm_Title" xml:space="preserve">
+    <value>确认</value>
+  </data>
+  <data name="Confirm_DeleteCard" xml:space="preserve">
+    <value>删除卡片“{0}”？</value>
+  </data>
+  <data name="Confirm_DeleteColumn" xml:space="preserve">
+    <value>删除列“{0}”及其中的 {1} 张卡片？</value>
+  </data>
+  <data name="Confirm_DiscardEdit" xml:space="preserve">
+    <value>放弃未保存的更改？</value>
+  </data>
+  <data name="Error_Column_TitleRequired" xml:space="preserve">
+    <value>列标题不能为空。</value>
+  </data>
+  <data name="Error_Column_Duplicate" xml:space="preserve">
+    <value>已存在名为“{0}”的列（名称不区分大小写）。</value>
+  </data>
+  <data name="Error_Tag_NameRequired" xml:space="preserve">
+    <value>标签名称不能为空。</value>
+  </data>
+  <data name="Error_Tag_Duplicate" xml:space="preserve">
+    <value>已存在名为“{0}”的标签（名称不区分大小写）。</value>
+  </data>
+  <data name="Error_Tag_InvalidColor" xml:space="preserve">
+    <value>颜色须为 #RRGGBB 格式。</value>
+  </data>
+  <data name="Error_SaveFailed" xml:space="preserve">
+    <value>保存失败：{0}</value>
+  </data>
+  <data name="Error_Operation_Title" xml:space="preserve">
+    <value>操作失败</value>
+  </data>
+  <data name="Error_Operation_Message" xml:space="preserve">
+    <value>更改未能写入工作区，看板显示的是已保存的状态。</value>
+  </data>
 </root>

YKanBan/Storage/StorageExceptions.cs

@@ -136,3 +136,69 @@ public sealed class SchemaVersionException : YKanBanException
         SupportedVersion = supportedVersion;
     }
 }
+
+/// <summary>
+/// The entities whose name/title is constrained by the schema to be non-empty
+/// and table-unique.
+/// </summary>
+public enum NamedEntity
+{
+    /// <summary>A board column (constraint on columns.title).</summary>
+    Column,
+
+    /// <summary>A tag (constraint on tags.name).</summary>
+    Tag,
+}
+
+/// <summary>
+/// Signals that a column title or tag name was empty, violating the schema's
+/// CHECK constraint. Nothing was written.
+/// </summary>
+public sealed class NameRequiredException : YKanBanException
+{
+    /// <summary>
+    /// Gets the entity whose name was empty.
+    /// </summary>
+    public NamedEntity Entity { get; }
+
+    /// <summary>
+    /// Initializes the exception for an empty name.
+    /// </summary>
+    /// <param name="entity">The entity whose name was empty.</param>
+    /// <param name="innerException">The underlying constraint violation.</param>
+    public NameRequiredException(NamedEntity entity, Exception innerException)
+        : base($"The {entity.ToString().ToLowerInvariant()} name must not be empty.", innerException)
+    {
+        Entity = entity;
+    }
+}
+
+/// <summary>
+/// Signals that a column title or tag name already exists (compared with ASCII
+/// case folding), violating the schema's UNIQUE constraint. Nothing was written.
+/// </summary>
+public sealed class DuplicateNameException : YKanBanException
+{
+    /// <summary>
+    /// Gets the entity whose name collided.
+    /// </summary>
+    public NamedEntity Entity { get; }
+
+    /// <summary>
+    /// Gets the rejected name.
+    /// </summary>
+    public string Name { get; }
+
+    /// <summary>
+    /// Initializes the exception for a name collision.
+    /// </summary>
+    /// <param name="entity">The entity whose name collided.</param>
+    /// <param name="name">The rejected name.</param>
+    /// <param name="innerException">The underlying constraint violation.</param>
+    public DuplicateNameException(NamedEntity entity, string name, Exception innerException)
+        : base($"A {entity.ToString().ToLowerInvariant()} named '{name}' already exists.", innerException)
+    {
+        Entity = entity;
+        Name = name;
+    }
+}

YKanBan/Storage/Workspace/WorkspaceRepository.cs

@@ -1,5 +1,6 @@
 using Microsoft.Data.Sqlite;
 using YKanBan.Models;
+using YKanBan.Search;
 
 namespace YKanBan.Storage.Workspace;
 
@@ -71,14 +72,15 @@ public sealed class WorkspaceRepository : IDisposable
     /// <param name="title">The non-empty, table-unique title.</param>
     /// <param name="description">The optional description.</param>
     /// <returns>The created column.</returns>
-    /// <exception cref="SqliteException">The title is empty or not unique.</exception>
+    /// <exception cref="NameRequiredException">The title is empty.</exception>
+    /// <exception cref="DuplicateNameException">The title is not unique.</exception>
     public ColumnModel AddColumn(string title, string description)
     {
         long now = UnixTime.Now;
         using var transaction = _connection.BeginTransaction();
-        long id = ExecuteInsert(transaction,
+        long id = MapNameConstraint(NamedEntity.Column, title, () => ExecuteInsert(transaction,
             "INSERT INTO columns (title, description, created_at, updated_at) VALUES ($title, $description, $now, $now);",
-            ("$title", title), ("$description", description), ("$now", now));
+            ("$title", title), ("$description", description), ("$now", now)));
         transaction.Commit();
 
         return new ColumnModel(id, title, description, now, now);
@@ -93,7 +95,8 @@ public sealed class WorkspaceRepository : IDisposable
     /// <param name="description">The new description.</param>
     /// <returns>The updated column, or the unchanged one for a no-op.</returns>
     /// <exception cref="InvalidOperationException">The column does not exist.</exception>
-    /// <exception cref="SqliteException">The new title is empty or not unique.</exception>
+    /// <exception cref="NameRequiredException">The new title is empty.</exception>
+    /// <exception cref="DuplicateNameException">The new title is not unique.</exception>
     public ColumnModel UpdateColumn(long columnId, string title, string description)
     {
         ColumnModel existing = GetColumn(columnId)
@@ -112,7 +115,7 @@ public sealed class WorkspaceRepository : IDisposable
         command.Parameters.AddWithValue("$description", description);
         command.Parameters.AddWithValue("$now", now);
         command.Parameters.AddWithValue("$id", columnId);
-        command.ExecuteNonQuery();
+        MapNameConstraint(NamedEntity.Column, title, command.ExecuteNonQuery);
 
         return new ColumnModel(columnId, title, description, existing.CreatedAt, now);
     }
@@ -132,16 +135,34 @@ public sealed class WorkspaceRepository : IDisposable
     /// Returns all cards with their assigned tags loaded, ordered by id ascending.
     /// </summary>
     /// <returns>All cards in id order.</returns>
-    public IReadOnlyList<CardModel> GetCards()
+    public IReadOnlyList<CardModel> GetCards() => GetCards(null);
+
+    /// <summary>
+    /// Returns the cards matching a compiled search, with all of their assigned
+    /// tags loaded (the filter selects cards, it never trims their tag lists),
+    /// ordered by id ascending.
+    /// </summary>
+    /// <param name="filter">The compiled search, or <see langword="null"/> for every card.</param>
+    /// <returns>The matching cards in id order.</returns>
+    public IReadOnlyList<CardModel> GetCards(CompiledSearch? filter)
     {
-        using var command = CreateCommand("""
+        string where = filter is null ? "" : $"WHERE {filter.Predicate}";
+        using var command = CreateCommand($"""
             SELECT c.id, c.column_id, c.title, c.content, c.created_at, c.updated_at,
                    t.id, t.name, t.color, t.description
             FROM cards c
             LEFT JOIN card_tags ct ON ct.card_id = c.id
             LEFT JOIN tags t ON t.id = ct.tag_id
-            ORDER BY c.id ASC;
+            {where}
+            ORDER BY c.id ASC, t.id ASC;
             """);
+        if (filter is not null)
+        {
+            foreach ((string name, object value) in filter.Parameters)
+            {
+                command.Parameters.AddWithValue(name, value);
+            }
+        }
         using var reader = command.ExecuteReader();
 
         // Single grouped pass: one output card per distinct card id, with its tags accumulated alongside.
@@ -186,7 +207,7 @@ public sealed class WorkspaceRepository : IDisposable
             LEFT JOIN card_tags ct ON ct.card_id = c.id
             LEFT JOIN tags t ON t.id = ct.tag_id
             WHERE c.id = $id
-            ORDER BY c.id ASC;
+            ORDER BY t.id ASC;
             """);
         command.Parameters.AddWithValue("$id", cardId);
         using var reader = command.ExecuteReader();
@@ -302,6 +323,18 @@ public sealed class WorkspaceRepository : IDisposable
         return command.ExecuteNonQuery() > 0;
     }
 
+    /// <summary>
+    /// Counts the cards of a column, regardless of any search filter.
+    /// </summary>
+    /// <param name="columnId">The column id.</param>
+    /// <returns>The number of cards in the column.</returns>
+    public long CountCards(long columnId)
+    {
+        using var command = CreateCommand("SELECT COUNT(*) FROM cards WHERE column_id = $id;");
+        command.Parameters.AddWithValue("$id", columnId);
+        return (long)command.ExecuteScalar()!;
+    }
+
     /// <summary>
     /// Deletes a card; the DDL cascade removes its tag links.
     /// </summary>
@@ -361,13 +394,14 @@ public sealed class WorkspaceRepository : IDisposable
     /// <param name="color">The tag color.</param>
     /// <param name="description">The optional description.</param>
     /// <returns>The created tag.</returns>
-    /// <exception cref="SqliteException">The name is empty or not unique.</exception>
+    /// <exception cref="NameRequiredException">The name is empty.</exception>
+    /// <exception cref="DuplicateNameException">The name is not unique.</exception>
     public TagModel AddTag(string name, RgbColor color, string description)
     {
         using var transaction = _connection.BeginTransaction();
-        long id = ExecuteInsert(transaction,
+        long id = MapNameConstraint(NamedEntity.Tag, name, () => ExecuteInsert(transaction,
             "INSERT INTO tags (name, color, description) VALUES ($name, $color, $description);",
-            ("$name", name), ("$color", color.ToString()), ("$description", description));
+            ("$name", name), ("$color", color.ToString()), ("$description", description)));
         transaction.Commit();
 
         return new TagModel(id, name, color, description);
@@ -382,7 +416,8 @@ public sealed class WorkspaceRepository : IDisposable
     /// <param name="description">The new description.</param>
     /// <returns>The updated tag, or the unchanged one for a no-op.</returns>
     /// <exception cref="InvalidOperationException">The tag does not exist.</exception>
-    /// <exception cref="SqliteException">The new name is empty or not unique.</exception>
+    /// <exception cref="NameRequiredException">The new name is empty.</exception>
+    /// <exception cref="DuplicateNameException">The new name is not unique.</exception>
     public TagModel UpdateTag(long tagId, string name, RgbColor color, string description)
     {
         TagModel existing = GetTag(tagId)
@@ -400,7 +435,7 @@ public sealed class WorkspaceRepository : IDisposable
         command.Parameters.AddWithValue("$color", color.ToString());
         command.Parameters.AddWithValue("$description", description);
         command.Parameters.AddWithValue("$id", tagId);
-        command.ExecuteNonQuery();
+        MapNameConstraint(NamedEntity.Tag, name, command.ExecuteNonQuery);
 
         return new TagModel(tagId, name, color, description);
     }
@@ -416,6 +451,40 @@ public sealed class WorkspaceRepository : IDisposable
 
     #region Helpers
 
+    /// <summary>SQLite extended result code of a failed CHECK constraint.</summary>
+    private const int SqliteConstraintCheck = 275;
+
+    /// <summary>SQLite extended result code of a failed UNIQUE constraint.</summary>
+    private const int SqliteConstraintUnique = 2067;
+
+    /// <summary>
+    /// Runs a write against columns.title or tags.name and translates the
+    /// schema's non-empty (CHECK) and uniqueness (UNIQUE) violations into typed
+    /// exceptions that the UI maps to localized messages.
+    /// </summary>
+    /// <typeparam name="T">The write's result type.</typeparam>
+    /// <param name="entity">The entity being written.</param>
+    /// <param name="name">The name or title being written.</param>
+    /// <param name="write">The write to run.</param>
+    /// <returns>The write's result.</returns>
+    /// <exception cref="NameRequiredException">The name is empty.</exception>
+    /// <exception cref="DuplicateNameException">The name is not unique.</exception>
+    private static T MapNameConstraint<T>(NamedEntity entity, string name, Func<T> write)
+    {
+        try
+        {
+            return write();
+        }
+        catch (SqliteException exception) when (exception.SqliteExtendedErrorCode == SqliteConstraintCheck)
+        {
+            throw new NameRequiredException(entity, exception);
+        }
+        catch (SqliteException exception) when (exception.SqliteExtendedErrorCode == SqliteConstraintUnique)
+        {
+            throw new DuplicateNameException(entity, name, exception);
+        }
+    }
+
     /// <summary>
     /// Returns the tag with the given id.
     /// </summary>

YKanBan/ViewModels/Board/BoardViewModel.cs

@@ -0,0 +1,270 @@
+using System.Collections.ObjectModel;
+using System.Diagnostics;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using YKanBan.Models;
+using YKanBan.Search;
+using YKanBan.Storage.AppData;
+using YKanBan.Storage.Workspace;
+using YKanBan.ViewModels.Dialogs;
+
+namespace YKanBan.ViewModels.Board;
+
+/// <summary>
+/// The board tab: the toolbar (new column, column width, card sort) and the
+/// columns laid out left to right, each listing its cards top to bottom.
+///
+/// Every write is followed by <see cref="Refresh"/>, which re-queries with the
+/// most recently successful search expression (<see cref="ActiveFilter"/>), not
+/// with whatever the search box currently holds: cards that no longer match
+/// disappear, new cards that do not match are not shown, and empty columns stay.
+/// </summary>
+public sealed partial class BoardViewModel : ViewModelBase
+{
+    private readonly WorkspaceRepository _repository;
+    private readonly AppConfig _config;
+    private readonly IDialogService _dialogs;
+
+    /// <summary>
+    /// Initializes the board and loads it.
+    /// </summary>
+    /// <param name="repository">The workspace repository.</param>
+    /// <param name="config">The live application configuration (sort, column width, confirmations).</param>
+    /// <param name="dialogs">The modal dialog service.</param>
+    public BoardViewModel(WorkspaceRepository repository, AppConfig config, IDialogService dialogs)
+    {
+        _repository = repository;
+        _config = config;
+        _dialogs = dialogs;
+
+        SortOptions =
+        [
+            new OptionItem(CardSortOption.Id, Resources.Sort_Id),
+            new OptionItem(CardSortOption.UpdatedAt, Resources.Sort_UpdatedAt),
+            new OptionItem(CardSortOption.Title, Resources.Sort_Title),
+        ];
+        selectedSort = SortOptions.FirstOrDefault(option => (CardSortOption)option.Value == config.Sort.Card)
+            ?? SortOptions[0];
+        columnWidth = ColumnWidths.Resolve(config.ColumnWidth);
+
+        Refresh();
+    }
+
+    /// <summary>
+    /// Gets the columns in id order.
+    /// </summary>
+    public ObservableCollection<ColumnViewModel> Columns { get; } = [];
+
+    /// <summary>
+    /// Gets the card sort options shown in the combobox.
+    /// </summary>
+    public IReadOnlyList<OptionItem> SortOptions { get; }
+
+    /// <summary>
+    /// Gets the column models in id order, as of the last refresh.
+    /// </summary>
+    public IReadOnlyList<ColumnModel> ColumnModels { get; private set; } = [];
+
+    /// <summary>
+    /// Gets or sets the most recently successful search; <see langword="null"/> shows every card.
+    /// </summary>
+    public CompiledSearch? ActiveFilter { get; set; }
+
+    /// <summary>
+    /// Gets or sets the selected card sort option; changing it is persisted and re-sorts the board.
+    /// </summary>
+    [ObservableProperty]
+    private OptionItem selectedSort;
+
+    /// <summary>
+    /// Gets the effective column width in pixels.
+    /// </summary>
+    [ObservableProperty]
+    private int columnWidth;
+
+    /// <summary>
+    /// Reloads the columns and the cards matching <see cref="ActiveFilter"/>.
+    /// </summary>
+    public void Refresh()
+    {
+        IReadOnlyList<ColumnModel> columns = _repository.GetColumns();
+        ILookup<long, CardModel> cardsByColumn = _repository.GetCards(ActiveFilter).ToLookup(card => card.ColumnId);
+        var sort = (CardSortOption)SelectedSort.Value;
+
+        ColumnModels = columns;
+        Columns.Clear();
+        foreach (ColumnModel column in columns)
+        {
+            Columns.Add(new ColumnViewModel(this, column, CardOrdering.Sort(cardsByColumn[column.Id], sort)));
+        }
+    }
+
+    /// <summary>
+    /// Opens the new-column dialog.
+    /// </summary>
+    /// <returns>A task completing when the dialog closes.</returns>
+    [RelayCommand]
+    private async Task AddColumnAsync()
+    {
+        var dialog = new ColumnEditorDialogViewModel(null, (title, description) => _repository.AddColumn(title, description));
+        if (await _dialogs.ShowAsync(dialog))
+        {
+            Refresh();
+        }
+    }
+
+    /// <summary>
+    /// Opens the column width dialog and applies the choice.
+    /// </summary>
+    /// <returns>A task completing when the dialog closes.</returns>
+    [RelayCommand]
+    private async Task AdjustColumnWidthAsync()
+    {
+        if (await _dialogs.ShowAsync(new ColumnWidthDialogViewModel(_config.ColumnWidth)))
+        {
+            ColumnWidth = ColumnWidths.Resolve(_config.ColumnWidth);
+        }
+    }
+
+    /// <summary>
+    /// Opens the edit-column dialog.
+    /// </summary>
+    /// <param name="column">The column to edit.</param>
+    /// <returns>A task completing when the dialog closes.</returns>
+    internal async Task EditColumnAsync(ColumnModel column)
+    {
+        var dialog = new ColumnEditorDialogViewModel(
+            column, (title, description) => _repository.UpdateColumn(column.Id, title, description));
+        if (await _dialogs.ShowAsync(dialog))
+        {
+            Refresh();
+        }
+    }
+
+    /// <summary>
+    /// Deletes a column with its cards, asking first when that confirmation is enabled.
+    /// </summary>
+    /// <param name="column">The column to delete.</param>
+    /// <returns>A task completing when the column is deleted or the deletion was declined.</returns>
+    internal async Task DeleteColumnAsync(ColumnModel column)
+    {
+        await RunWriteAsync(async () =>
+        {
+            if (_config.Confirmations.DeleteColumn)
+            {
+                long count = _repository.CountCards(column.Id);
+                string message = string.Format(Resources.Culture, Resources.Confirm_DeleteColumn, column.Title, count);
+                if (!await _dialogs.ConfirmAsync(message, Resources.Common_Delete))
+                {
+                    return false;
+                }
+            }
+            _repository.DeleteColumn(column.Id);
+            return true;
+        });
+    }
+
+    /// <summary>
+    /// Opens the card editor for a new card in a column.
+    /// </summary>
+    /// <param name="columnId">The column receiving the card.</param>
+    /// <returns>A task completing when the editor closes.</returns>
+    internal async Task AddCardAsync(long columnId)
+    {
+        var editor = CardEditorDialogViewModel.ForNewCard(
+            _repository, _dialogs, columnId, _config.Confirmations.DiscardEdit);
+        await _dialogs.ShowAsync(editor);
+
+        // Refresh even after a cancel: the nested picker may have created tags.
+        Refresh();
+    }
+
+    /// <summary>
+    /// Opens the card editor for an existing card, reloaded from the database.
+    /// </summary>
+    /// <param name="cardId">The card to edit.</param>
+    /// <returns>A task completing when the editor closes.</returns>
+    internal async Task EditCardAsync(long cardId)
+    {
+        CardModel? card = _repository.GetCard(cardId);
+        if (card is null)
+        {
+            Refresh();
+            return;
+        }
+
+        var editor = CardEditorDialogViewModel.ForExistingCard(
+            _repository, _dialogs, card, _config.Confirmations.DiscardEdit);
+        await _dialogs.ShowAsync(editor);
+        Refresh();
+    }
+
+    /// <summary>
+    /// Deletes a card, asking first when that confirmation is enabled.
+    /// </summary>
+    /// <param name="card">The card to delete.</param>
+    /// <returns>A task completing when the card is deleted or the deletion was declined.</returns>
+    internal async Task DeleteCardAsync(CardModel card)
+    {
+        await RunWriteAsync(async () =>
+        {
+            if (_config.Confirmations.DeleteCard)
+            {
+                string message = string.Format(Resources.Culture, Resources.Confirm_DeleteCard, CardViewModel.FormatHeader(card));
+                if (!await _dialogs.ConfirmAsync(message, Resources.Common_Delete))
+                {
+                    return false;
+                }
+            }
+            _repository.DeleteCard(card.Id);
+            return true;
+        });
+    }
+
+    /// <summary>
+    /// Moves a card to another column.
+    /// </summary>
+    /// <param name="cardId">The card to move.</param>
+    /// <param name="columnId">The target column.</param>
+    /// <returns>A task completing when the card is moved.</returns>
+    internal Task MoveCardAsync(long cardId, long columnId) =>
+        RunWriteAsync(() =>
+        {
+            _repository.MoveCard(cardId, columnId);
+            return Task.FromResult(true);
+        });
+
+    /// <summary>
+    /// Runs an immediate write and refreshes after it; an unexpected failure is
+    /// reported in a message dialog and the board is refreshed to the stored state.
+    /// </summary>
+    /// <param name="write">The write; returns <see langword="false"/> when the user declined it.</param>
+    /// <returns>A task completing after the refresh.</returns>
+    private async Task RunWriteAsync(Func<Task<bool>> write)
+    {
+        try
+        {
+            if (!await write())
+            {
+                return;
+            }
+        }
+        catch (Exception exception)
+        {
+            Debug.WriteLine(exception);
+            await _dialogs.ShowMessageAsync(
+                Resources.Error_Operation_Title, Resources.Error_Operation_Message, exception.Message);
+        }
+        Refresh();
+    }
+
+    /// <summary>
+    /// Persists the sort option and re-sorts the board.
+    /// </summary>
+    /// <param name="value">The new sort option.</param>
+    partial void OnSelectedSortChanged(OptionItem value)
+    {
+        _config.Sort.Card = (CardSortOption)value.Value;
+        Refresh();
+    }
+}

YKanBan/ViewModels/Board/CardViewModel.cs

@@ -0,0 +1,141 @@
+using System.Globalization;
+using CommunityToolkit.Mvvm.Input;
+using YKanBan.Models;
+
+namespace YKanBan.ViewModels.Board;
+
+/// <summary>
+/// One card in a column list: "#id" in bold on the same line as the title, the
+/// content cut to a fixed length, and the tag badges below. No timestamps.
+/// Clicking the card opens the editor; its menu deletes or moves it.
+/// </summary>
+public sealed partial class CardViewModel : ViewModelBase
+{
+    /// <summary>
+    /// The number of text elements of content shown on the card.
+    /// </summary>
+    public const int ExcerptLength = 200;
+
+    private readonly BoardViewModel _board;
+
+    /// <summary>
+    /// Initializes the card.
+    /// </summary>
+    /// <param name="board">The owning board.</param>
+    /// <param name="card">The card shown.</param>
+    public CardViewModel(BoardViewModel board, CardModel card)
+    {
+        _board = board;
+        Card = card;
+        IdText = $"#{card.Id.ToString(CultureInfo.InvariantCulture)}";
+        Excerpt = MakeExcerpt(card.Content);
+        Tags = card.Tags.Select(tag => new TagBadgeViewModel(tag)).ToList();
+    }
+
+    /// <summary>
+    /// Gets the card shown.
+    /// </summary>
+    public CardModel Card { get; }
+
+    /// <summary>
+    /// Gets the "#id" text.
+    /// </summary>
+    public string IdText { get; }
+
+    /// <summary>
+    /// Gets the title; empty when the card has none.
+    /// </summary>
+    public string Title => Card.Title;
+
+    /// <summary>
+    /// Gets the title as it follows "#id" on the header line: a space and the
+    /// title, or nothing when the card has no title.
+    /// </summary>
+    public string TitleSuffix => Card.Title.Length == 0 ? "" : " " + Card.Title;
+
+    /// <summary>
+    /// Gets the truncated content.
+    /// </summary>
+    public string Excerpt { get; }
+
+    /// <summary>
+    /// Gets whether there is content to show.
+    /// </summary>
+    public bool HasExcerpt => Excerpt.Length > 0;
+
+    /// <summary>
+    /// Gets the tag badges.
+    /// </summary>
+    public IReadOnlyList<TagBadgeViewModel> Tags { get; }
+
+    /// <summary>
+    /// Gets whether the card has tags.
+    /// </summary>
+    public bool HasTags => Tags.Count > 0;
+
+    /// <summary>
+    /// Gets the "move to column" targets; built when the menu asks for them.
+    /// The card's current column is listed but disabled.
+    /// </summary>
+    public IReadOnlyList<MoveTargetViewModel> MoveTargets =>
+        _board.ColumnModels
+            .Select(column => new MoveTargetViewModel(column.Title, column.Id != Card.ColumnId, MoveToCommand, column.Id))
+            .ToList();
+
+    /// <summary>
+    /// Formats the "#id title" header text of a card ("#id" alone without a title).
+    /// </summary>
+    /// <param name="card">The card.</param>
+    /// <returns>The header text.</returns>
+    public static string FormatHeader(CardModel card) =>
+        card.Title.Length == 0
+            ? $"#{card.Id.ToString(CultureInfo.InvariantCulture)}"
+            : $"#{card.Id.ToString(CultureInfo.InvariantCulture)} {card.Title}";
+
+    /// <summary>
+    /// Cuts content to <see cref="ExcerptLength"/> text elements, appending an
+    /// ellipsis when something was cut. Surrogate pairs and combining sequences
+    /// are never split.
+    /// </summary>
+    /// <param name="content">The card content.</param>
+    /// <returns>The excerpt.</returns>
+    public static string MakeExcerpt(string content)
+    {
+        string trimmed = content.Trim();
+        var info = new StringInfo(trimmed);
+        return info.LengthInTextElements <= ExcerptLength
+            ? trimmed
+            : info.SubstringByTextElements(0, ExcerptLength).TrimEnd() + "…";
+    }
+
+    /// <summary>
+    /// Opens the card editor.
+    /// </summary>
+    /// <returns>A task completing when the editor closes.</returns>
+    [RelayCommand]
+    private Task EditAsync() => _board.EditCardAsync(Card.Id);
+
+    /// <summary>
+    /// Deletes the card.
+    /// </summary>
+    /// <returns>A task completing when the card is deleted or the deletion was declined.</returns>
+    [RelayCommand]
+    private Task DeleteAsync() => _board.DeleteCardAsync(Card);
+
+    /// <summary>
+    /// Moves the card to another column.
+    /// </summary>
+    /// <param name="columnId">The target column.</param>
+    /// <returns>A task completing when the card is moved.</returns>
+    [RelayCommand]
+    private Task MoveToAsync(long columnId) => _board.MoveCardAsync(Card.Id, columnId);
+}
+
+/// <summary>
+/// An entry of a card's "move to column" submenu.
+/// </summary>
+/// <param name="Title">The column title.</param>
+/// <param name="IsEnabled">Whether the entry can be picked (false for the card's own column).</param>
+/// <param name="Command">The move command.</param>
+/// <param name="ColumnId">The target column, passed to <paramref name="Command"/>.</param>
+public sealed record MoveTargetViewModel(string Title, bool IsEnabled, IAsyncRelayCommand<long> Command, long ColumnId);

YKanBan/ViewModels/Board/ColumnViewModel.cs

@@ -0,0 +1,77 @@
+using CommunityToolkit.Mvvm.Input;
+using YKanBan.Models;
+
+namespace YKanBan.ViewModels.Board;
+
+/// <summary>
+/// One board column: its header (title, card count, description, the new-card
+/// button and the edit/delete menu) and its virtualized card list.
+/// </summary>
+public sealed partial class ColumnViewModel : ViewModelBase
+{
+    private readonly BoardViewModel _board;
+
+    /// <summary>
+    /// Initializes the column.
+    /// </summary>
+    /// <param name="board">The owning board.</param>
+    /// <param name="column">The column shown.</param>
+    /// <param name="cards">The column's visible cards, in display order.</param>
+    public ColumnViewModel(BoardViewModel board, ColumnModel column, IReadOnlyList<CardModel> cards)
+    {
+        _board = board;
+        Column = column;
+        Cards = cards.Select(card => new CardViewModel(board, card)).ToList();
+    }
+
+    /// <summary>
+    /// Gets the column shown.
+    /// </summary>
+    public ColumnModel Column { get; }
+
+    /// <summary>
+    /// Gets the column title.
+    /// </summary>
+    public string Title => Column.Title;
+
+    /// <summary>
+    /// Gets the column description.
+    /// </summary>
+    public string Description => Column.Description;
+
+    /// <summary>
+    /// Gets whether the column has a description to show.
+    /// </summary>
+    public bool HasDescription => Column.Description.Length > 0;
+
+    /// <summary>
+    /// Gets the number of visible cards.
+    /// </summary>
+    public int CardCount => Cards.Count;
+
+    /// <summary>
+    /// Gets the visible cards in display order.
+    /// </summary>
+    public IReadOnlyList<CardViewModel> Cards { get; }
+
+    /// <summary>
+    /// Opens the card editor for a new card in this column.
+    /// </summary>
+    /// <returns>A task completing when the editor closes.</returns>
+    [RelayCommand]
+    private Task AddCardAsync() => _board.AddCardAsync(Column.Id);
+
+    /// <summary>
+    /// Opens the edit-column dialog.
+    /// </summary>
+    /// <returns>A task completing when the dialog closes.</returns>
+    [RelayCommand]
+    private Task EditAsync() => _board.EditColumnAsync(Column);
+
+    /// <summary>
+    /// Deletes the column with its cards.
+    /// </summary>
+    /// <returns>A task completing when the column is deleted or the deletion was declined.</returns>
+    [RelayCommand]
+    private Task DeleteAsync() => _board.DeleteColumnAsync(Column);
+}

YKanBan/ViewModels/Board/TagBadgeViewModel.cs

@@ -0,0 +1,57 @@
+using Avalonia.Media;
+using Avalonia.Media.Immutable;
+using YKanBan.Models;
+
+namespace YKanBan.ViewModels.Board;
+
+/// <summary>
+/// A colored tag badge (color + name). Tag colors are user data: they are shown
+/// as the literal #RRGGBB value in both themes, with a black or white text color
+/// chosen for contrast.
+/// </summary>
+public sealed class TagBadgeViewModel : ViewModelBase
+{
+    /// <summary>
+    /// Initializes the badge.
+    /// </summary>
+    /// <param name="tag">The tag shown.</param>
+    public TagBadgeViewModel(TagModel tag)
+    {
+        Tag = tag;
+        Background = new ImmutableSolidColorBrush(Color.FromRgb(tag.Color.R, tag.Color.G, tag.Color.B));
+        Foreground = IsLight(tag.Color) ? Brushes.Black : Brushes.White;
+    }
+
+    /// <summary>
+    /// Gets the tag shown.
+    /// </summary>
+    public TagModel Tag { get; }
+
+    /// <summary>
+    /// Gets the tag name.
+    /// </summary>
+    public string Name => Tag.Name;
+
+    /// <summary>
+    /// Gets the tag description.
+    /// </summary>
+    public string Description => Tag.Description;
+
+    /// <summary>
+    /// Gets the badge fill: the tag's own color.
+    /// </summary>
+    public IBrush Background { get; }
+
+    /// <summary>
+    /// Gets the text color contrasting with <see cref="Background"/>.
+    /// </summary>
+    public IBrush Foreground { get; }
+
+    /// <summary>
+    /// Decides whether dark text reads better on a color, by its relative luminance.
+    /// </summary>
+    /// <param name="color">The background color.</param>
+    /// <returns><see langword="true"/> for light colors.</returns>
+    public static bool IsLight(RgbColor color) =>
+        (0.299 * color.R) + (0.587 * color.G) + (0.114 * color.B) > 150;
+}

YKanBan/ViewModels/Dialogs/CardEditorDialogViewModel.cs

@@ -0,0 +1,228 @@
+using System.Collections.ObjectModel;
+using System.Globalization;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using YKanBan.Models;
+using YKanBan.Storage.Workspace;
+using YKanBan.ViewModels.Board;
+
+namespace YKanBan.ViewModels.Dialogs;
+
+/// <summary>
+/// The card editor — one unit of work. Title, content and the tag assignment
+/// are edited in memory and written together in a single transaction on
+/// confirmation; cancelling writes nothing about the card or its assignment.
+/// Confirming an existing card without effective changes writes nothing and
+/// keeps its timestamps. The only immediate write is the creation of a new tag
+/// in the nested tag picker (see <see cref="TagPickerDialogViewModel"/>).
+/// </summary>
+public sealed partial class CardEditorDialogViewModel : DialogViewModelBase
+{
+    private readonly WorkspaceRepository _repository;
+    private readonly IDialogService _dialogs;
+    private readonly CardModel? _existing;
+    private readonly long _columnId;
+    private readonly bool _confirmDiscard;
+    private readonly string _initialTitle;
+    private readonly string _initialContent;
+    private readonly HashSet<long> _initialTagIds;
+
+    /// <summary>
+    /// Initializes the editor for a new card in a column.
+    /// </summary>
+    /// <param name="repository">The workspace repository.</param>
+    /// <param name="dialogs">The dialog service, for the nested tag picker and the discard confirmation.</param>
+    /// <param name="columnId">The column receiving the new card.</param>
+    /// <param name="confirmDiscard">Whether discarding changes asks for confirmation.</param>
+    /// <returns>The editor.</returns>
+    public static CardEditorDialogViewModel ForNewCard(
+        WorkspaceRepository repository, IDialogService dialogs, long columnId, bool confirmDiscard) =>
+        new(repository, dialogs, null, columnId, confirmDiscard);
+
+    /// <summary>
+    /// Initializes the editor for an existing card.
+    /// </summary>
+    /// <param name="repository">The workspace repository.</param>
+    /// <param name="dialogs">The dialog service, for the nested tag picker and the discard confirmation.</param>
+    /// <param name="card">The card to edit.</param>
+    /// <param name="confirmDiscard">Whether discarding changes asks for confirmation.</param>
+    /// <returns>The editor.</returns>
+    public static CardEditorDialogViewModel ForExistingCard(
+        WorkspaceRepository repository, IDialogService dialogs, CardModel card, bool confirmDiscard) =>
+        new(repository, dialogs, card, card.ColumnId, confirmDiscard);
+
+    private CardEditorDialogViewModel(
+        WorkspaceRepository repository, IDialogService dialogs, CardModel? existing, long columnId, bool confirmDiscard)
+    {
+        _repository = repository;
+        _dialogs = dialogs;
+        _existing = existing;
+        _columnId = columnId;
+        _confirmDiscard = confirmDiscard;
+        _initialTitle = existing?.Title ?? "";
+        _initialContent = existing?.Content ?? "";
+        _initialTagIds = existing?.Tags.Select(tag => tag.Id).ToHashSet() ?? [];
+
+        cardTitle = _initialTitle;
+        content = _initialContent;
+        Tags = new ObservableCollection<TagBadgeViewModel>(
+            existing?.Tags.Select(tag => new TagBadgeViewModel(tag)) ?? []);
+
+        if (existing is null)
+        {
+            Title = Resources.CardEditor_TitleNew;
+        }
+        else
+        {
+            CultureInfo? culture = Resources.Culture;
+            Title = Resources.CardEditor_TitleEdit;
+            IdText = $"#{existing.Id.ToString(CultureInfo.InvariantCulture)}";
+            CreatedText = string.Format(culture, Resources.CardEditor_Created, FormatTime(existing.CreatedAt));
+            ModifiedText = string.Format(culture, Resources.CardEditor_Modified, FormatTime(existing.UpdatedAt));
+        }
+    }
+
+    /// <summary>
+    /// Gets the localized window title.
+    /// </summary>
+    public string Title { get; }
+
+    /// <summary>
+    /// Gets whether the card already exists (and thus has an id and timestamps to show).
+    /// </summary>
+    public bool IsExisting => _existing is not null;
+
+    /// <summary>
+    /// Gets the read-only "#id" text of an existing card.
+    /// </summary>
+    public string? IdText { get; }
+
+    /// <summary>
+    /// Gets the read-only creation time line of an existing card.
+    /// </summary>
+    public string? CreatedText { get; }
+
+    /// <summary>
+    /// Gets the read-only modification time line of an existing card.
+    /// </summary>
+    public string? ModifiedText { get; }
+
+    /// <summary>
+    /// Gets the tags assigned in this edit session, in assignment order.
+    /// </summary>
+    public ObservableCollection<TagBadgeViewModel> Tags { get; }
+
+    /// <summary>
+    /// Gets the card written on confirmation; <see langword="null"/> until then.
+    /// </summary>
+    public CardModel? SavedCard { get; private set; }
+
+    /// <summary>
+    /// Gets or sets the card title; the empty string means "no title".
+    /// </summary>
+    [ObservableProperty]
+    private string cardTitle;
+
+    /// <summary>
+    /// Gets or sets the multiline card content.
+    /// </summary>
+    [ObservableProperty]
+    private string content;
+
+    /// <summary>
+    /// Gets the localized error of the last confirmation, or <see langword="null"/>.
+    /// </summary>
+    [ObservableProperty]
+    [NotifyPropertyChangedFor(nameof(HasError))]
+    private string? errorMessage;
+
+    /// <summary>
+    /// Gets whether an error is shown.
+    /// </summary>
+    public bool HasError => ErrorMessage is not null;
+
+    /// <summary>
+    /// Gets whether the in-memory state differs from the state the editor opened with.
+    /// </summary>
+    public bool IsDirty =>
+        CardTitle != _initialTitle
+        || Content != _initialContent
+        || !_initialTagIds.SetEquals(Tags.Select(tag => tag.Tag.Id));
+
+    /// <summary>
+    /// Opens the nested tag picker and assigns the picked (or newly created) tag.
+    /// </summary>
+    /// <returns>A task completing when the picker closes.</returns>
+    [RelayCommand]
+    private async Task AddTagAsync()
+    {
+        var picker = new TagPickerDialogViewModel(_repository, Tags.Select(tag => tag.Tag.Id).ToHashSet());
+        if (await _dialogs.ShowAsync(picker) && picker.PickedTag is { } picked)
+        {
+            Tags.Add(new TagBadgeViewModel(picked));
+        }
+    }
+
+    /// <summary>
+    /// Removes a tag from the in-memory assignment.
+    /// </summary>
+    /// <param name="tag">The tag to remove.</param>
+    [RelayCommand]
+    private void RemoveTag(TagBadgeViewModel tag) => Tags.Remove(tag);
+
+    /// <summary>
+    /// Writes the card in one transaction and closes; an unchanged existing card
+    /// is not written at all.
+    /// </summary>
+    [RelayCommand]
+    private void Confirm()
+    {
+        if (_existing is not null && !IsDirty)
+        {
+            SavedCard = _existing;
+            Close(true);
+            return;
+        }
+
+        long[] tagIds = Tags.Select(tag => tag.Tag.Id).ToArray();
+        try
+        {
+            SavedCard = _existing is null
+                ? _repository.AddCard(_columnId, CardTitle, Content, tagIds)
+                : _repository.UpdateCard(_existing.Id, CardTitle, Content, tagIds);
+        }
+        catch (Exception exception)
+        {
+            ErrorMessage = WriteErrorMessages.Describe(exception);
+            return;
+        }
+        Close(true);
+    }
+
+    /// <summary>
+    /// Closes without writing the card, asking first when there are unsaved
+    /// changes and the discard confirmation is enabled.
+    /// </summary>
+    [RelayCommand]
+    private Task CancelAsync() => RequestCancelAsync();
+
+    /// <inheritdoc/>
+    public override async Task RequestCancelAsync()
+    {
+        if (_confirmDiscard && IsDirty
+            && !await _dialogs.ConfirmAsync(Resources.Confirm_DiscardEdit, Resources.Common_Discard))
+        {
+            return;
+        }
+        Close(false);
+    }
+
+    /// <summary>
+    /// Formats a Unix-seconds timestamp in local time with the language's pattern.
+    /// </summary>
+    /// <param name="unixSeconds">The timestamp.</param>
+    /// <returns>The formatted time.</returns>
+    private static string FormatTime(long unixSeconds) =>
+        DateTimeOffset.FromUnixTimeSeconds(unixSeconds).ToLocalTime()
+            .ToString(Resources.Common_TimePattern, Resources.Culture);
+}

YKanBan/ViewModels/Dialogs/ColumnEditorDialogViewModel.cs

@@ -0,0 +1,82 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using YKanBan.Models;
+
+namespace YKanBan.ViewModels.Dialogs;
+
+/// <summary>
+/// The new-column / edit-column dialog: a required title and an optional
+/// description. Confirming writes through the supplied save callback; a
+/// schema-constraint violation (empty or duplicate title) is shown inside the
+/// dialog, which stays open with the user's input intact.
+/// </summary>
+public sealed partial class ColumnEditorDialogViewModel : DialogViewModelBase
+{
+    private readonly Action<string, string> _save;
+
+    /// <summary>
+    /// Initializes the dialog.
+    /// </summary>
+    /// <param name="existing">The column being edited, or <see langword="null"/> for a new column.</param>
+    /// <param name="save">Writes the title and description; throws the repository's typed exceptions.</param>
+    public ColumnEditorDialogViewModel(ColumnModel? existing, Action<string, string> save)
+    {
+        _save = save;
+        Title = existing is null ? Resources.ColumnEditor_TitleNew : Resources.ColumnEditor_TitleEdit;
+        columnTitle = existing?.Title ?? "";
+        description = existing?.Description ?? "";
+    }
+
+    /// <summary>
+    /// Gets the localized window title.
+    /// </summary>
+    public string Title { get; }
+
+    /// <summary>
+    /// Gets or sets the column title being edited (not trimmed).
+    /// </summary>
+    [ObservableProperty]
+    private string columnTitle;
+
+    /// <summary>
+    /// Gets or sets the column description being edited.
+    /// </summary>
+    [ObservableProperty]
+    private string description;
+
+    /// <summary>
+    /// Gets the localized constraint error of the last confirmation, or <see langword="null"/>.
+    /// </summary>
+    [ObservableProperty]
+    [NotifyPropertyChangedFor(nameof(HasError))]
+    private string? errorMessage;
+
+    /// <summary>
+    /// Gets whether an error is shown.
+    /// </summary>
+    public bool HasError => ErrorMessage is not null;
+
+    /// <summary>
+    /// Writes the column; closes on success, otherwise shows the constraint error.
+    /// </summary>
+    [RelayCommand]
+    private void Confirm()
+    {
+        try
+        {
+            _save(ColumnTitle, Description);
+        }
+        catch (Exception exception)
+        {
+            ErrorMessage = WriteErrorMessages.Describe(exception);
+            return;
+        }
+        Close(true);
+    }
+
+    /// <summary>
+    /// Closes without writing.
+    /// </summary>
+    [RelayCommand]
+    private Task CancelAsync() => RequestCancelAsync();
+}

YKanBan/ViewModels/Dialogs/ColumnWidthDialogViewModel.cs

@@ -0,0 +1,129 @@
+using System.Globalization;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using YKanBan.Storage.AppData;
+
+namespace YKanBan.ViewModels.Dialogs;
+
+/// <summary>
+/// The column width dialog: one of the narrow / standard / wide presets or a
+/// manual pixel width (160–720). The choice is global and is committed to the
+/// live configuration on confirmation only.
+/// </summary>
+public sealed partial class ColumnWidthDialogViewModel : DialogViewModelBase
+{
+    private readonly ColumnWidthSettings _settings;
+
+    /// <summary>
+    /// Initializes the dialog from the live settings.
+    /// </summary>
+    /// <param name="settings">The live column width settings.</param>
+    public ColumnWidthDialogViewModel(ColumnWidthSettings settings)
+    {
+        _settings = settings;
+        CultureInfo? culture = Resources.Culture;
+        NarrowLabel = string.Format(culture, Resources.ColumnWidth_Narrow, ColumnWidths.Narrow);
+        StandardLabel = string.Format(culture, Resources.ColumnWidth_Standard, ColumnWidths.Standard);
+        WideLabel = string.Format(culture, Resources.ColumnWidth_Wide, ColumnWidths.Wide);
+        RangeLabel = string.Format(culture, Resources.ColumnWidth_Range, AppConfig.MinColumnPixels, AppConfig.MaxColumnPixels);
+
+        isNarrow = settings.Preset == ColumnWidthPreset.Narrow;
+        isStandard = settings.Preset == ColumnWidthPreset.Standard;
+        isWide = settings.Preset == ColumnWidthPreset.Wide;
+        isCustom = settings.Preset == ColumnWidthPreset.Custom;
+        customPixels = settings.CustomPixels;
+    }
+
+    /// <summary>Gets the localized narrow preset label.</summary>
+    public string NarrowLabel { get; }
+
+    /// <summary>Gets the localized standard preset label.</summary>
+    public string StandardLabel { get; }
+
+    /// <summary>Gets the localized wide preset label.</summary>
+    public string WideLabel { get; }
+
+    /// <summary>Gets the localized accepted pixel range.</summary>
+    public string RangeLabel { get; }
+
+    /// <summary>Gets the smallest accepted custom width.</summary>
+    public int MinPixels => AppConfig.MinColumnPixels;
+
+    /// <summary>Gets the largest accepted custom width.</summary>
+    public int MaxPixels => AppConfig.MaxColumnPixels;
+
+    /// <summary>Gets or sets whether the narrow preset is selected.</summary>
+    [ObservableProperty]
+    private bool isNarrow;
+
+    /// <summary>Gets or sets whether the standard preset is selected.</summary>
+    [ObservableProperty]
+    private bool isStandard;
+
+    /// <summary>Gets or sets whether the wide preset is selected.</summary>
+    [ObservableProperty]
+    private bool isWide;
+
+    /// <summary>Gets or sets whether the manual width is selected.</summary>
+    [ObservableProperty]
+    private bool isCustom;
+
+    /// <summary>Gets or sets the manual width in pixels; <see langword="null"/> while the box is empty.</summary>
+    [ObservableProperty]
+    private decimal? customPixels;
+
+    /// <summary>
+    /// Commits the selection to the live settings and closes.
+    /// </summary>
+    [RelayCommand]
+    private void Confirm()
+    {
+        if (IsCustom)
+        {
+            int pixels = (int)Math.Clamp(CustomPixels ?? _settings.CustomPixels, AppConfig.MinColumnPixels, AppConfig.MaxColumnPixels);
+            _settings.Preset = ColumnWidthPreset.Custom;
+            _settings.CustomPixels = pixels;
+        }
+        else
+        {
+            _settings.Preset = IsNarrow ? ColumnWidthPreset.Narrow
+                : IsWide ? ColumnWidthPreset.Wide
+                : ColumnWidthPreset.Standard;
+        }
+        Close(true);
+    }
+
+    /// <summary>
+    /// Closes without changing anything.
+    /// </summary>
+    [RelayCommand]
+    private Task CancelAsync() => RequestCancelAsync();
+}
+
+/// <summary>
+/// Pixel widths of the column width presets.
+/// </summary>
+public static class ColumnWidths
+{
+    /// <summary>The narrow preset width.</summary>
+    public const int Narrow = 240;
+
+    /// <summary>The standard (default) preset width.</summary>
+    public const int Standard = 320;
+
+    /// <summary>The wide preset width.</summary>
+    public const int Wide = 480;
+
+    /// <summary>
+    /// Resolves the effective column width of the settings.
+    /// </summary>
+    /// <param name="settings">The column width settings.</param>
+    /// <returns>The width in pixels.</returns>
+    public static int Resolve(ColumnWidthSettings settings) => settings.Preset switch
+    {
+        ColumnWidthPreset.Narrow => Narrow,
+        ColumnWidthPreset.Wide => Wide,
+        ColumnWidthPreset.Custom => Math.Clamp(settings.CustomPixels, AppConfig.MinColumnPixels, AppConfig.MaxColumnPixels),
+        _ => Standard,
+    };
+}

YKanBan/ViewModels/Dialogs/ConfirmDialogViewModel.cs

@@ -0,0 +1,43 @@
+using CommunityToolkit.Mvvm.Input;
+
+namespace YKanBan.ViewModels.Dialogs;
+
+/// <summary>
+/// A yes/no confirmation: a message, a confirming button with an action
+/// specific label (for example "Delete") and a cancel button.
+/// </summary>
+public sealed partial class ConfirmDialogViewModel : DialogViewModelBase
+{
+    /// <summary>
+    /// Initializes the confirmation.
+    /// </summary>
+    /// <param name="message">The localized question.</param>
+    /// <param name="confirmLabel">The localized label of the confirming button.</param>
+    public ConfirmDialogViewModel(string message, string confirmLabel)
+    {
+        Message = message;
+        ConfirmLabel = confirmLabel;
+    }
+
+    /// <summary>
+    /// Gets the localized question.
+    /// </summary>
+    public string Message { get; }
+
+    /// <summary>
+    /// Gets the localized label of the confirming button.
+    /// </summary>
+    public string ConfirmLabel { get; }
+
+    /// <summary>
+    /// Closes with a positive result.
+    /// </summary>
+    [RelayCommand]
+    private void Confirm() => Close(true);
+
+    /// <summary>
+    /// Closes with a negative result.
+    /// </summary>
+    [RelayCommand]
+    private Task CancelAsync() => RequestCancelAsync();
+}

YKanBan/ViewModels/Dialogs/DialogViewModelBase.cs

@@ -0,0 +1,33 @@
+namespace YKanBan.ViewModels.Dialogs;
+
+/// <summary>
+/// Base class for modal dialog view models shown through <see cref="IDialogService"/>.
+/// The dialog decides when it closes: it raises <see cref="CloseRequested"/>
+/// with its result, and a close attempted by the window itself (title-bar close
+/// button, Escape) is routed to <see cref="RequestCancelAsync"/> so that the
+/// dialog can veto it, for example to ask before discarding edits.
+/// </summary>
+public abstract class DialogViewModelBase : ViewModelBase
+{
+    /// <summary>
+    /// Raised when the dialog wants to close; the argument is the dialog result.
+    /// </summary>
+    public event EventHandler<bool>? CloseRequested;
+
+    /// <summary>
+    /// Handles a cancel request coming from the window (or a cancel button).
+    /// The default closes immediately with a negative result.
+    /// </summary>
+    /// <returns>A task completing once the request has been handled.</returns>
+    public virtual Task RequestCancelAsync()
+    {
+        Close(false);
+        return Task.CompletedTask;
+    }
+
+    /// <summary>
+    /// Asks the hosting window to close with the given result.
+    /// </summary>
+    /// <param name="result">The dialog result.</param>
+    protected void Close(bool result) => CloseRequested?.Invoke(this, result);
+}

YKanBan/ViewModels/Dialogs/IDialogService.cs

@@ -0,0 +1,43 @@
+namespace YKanBan.ViewModels.Dialogs;
+
+/// <summary>
+/// Opens modal dialogs on behalf of view models, which never reference views.
+/// Every dialog is owned by the top-most open window, so a dialog opened from
+/// inside another dialog (nested modal) is centered on and blocks that dialog.
+/// </summary>
+public interface IDialogService
+{
+    /// <summary>
+    /// Shows a dialog modally and waits until it closes.
+    /// </summary>
+    /// <param name="dialog">The dialog view model; its window is resolved by naming convention.</param>
+    /// <returns>The dialog result: <see langword="true"/> when it was accepted.</returns>
+    Task<bool> ShowAsync(DialogViewModelBase dialog);
+}
+
+/// <summary>
+/// Convenience wrappers for the plain confirmation and message dialogs.
+/// </summary>
+public static class DialogServiceExtensions
+{
+    /// <summary>
+    /// Asks a yes/no question.
+    /// </summary>
+    /// <param name="dialogs">The dialog service.</param>
+    /// <param name="message">The localized question.</param>
+    /// <param name="confirmLabel">The localized label of the confirming button.</param>
+    /// <returns><see langword="true"/> when the user confirmed.</returns>
+    public static Task<bool> ConfirmAsync(this IDialogService dialogs, string message, string confirmLabel) =>
+        dialogs.ShowAsync(new ConfirmDialogViewModel(message, confirmLabel));
+
+    /// <summary>
+    /// Shows an informational message with an optional read-only details block.
+    /// </summary>
+    /// <param name="dialogs">The dialog service.</param>
+    /// <param name="title">The localized window title.</param>
+    /// <param name="message">The localized message.</param>
+    /// <param name="details">Untranslated diagnostic details, or <see langword="null"/>.</param>
+    /// <returns>A task completing when the dialog closes.</returns>
+    public static Task ShowMessageAsync(this IDialogService dialogs, string title, string message, string? details = null) =>
+        dialogs.ShowAsync(new MessageDialogViewModel(title, message, details));
+}

YKanBan/ViewModels/Dialogs/MessageDialogViewModel.cs

@@ -0,0 +1,49 @@
+using CommunityToolkit.Mvvm.Input;
+
+namespace YKanBan.ViewModels.Dialogs;
+
+/// <summary>
+/// A purely informational dialog with a single OK button: a localized message
+/// plus optional untranslated, copyable diagnostic details.
+/// </summary>
+public sealed partial class MessageDialogViewModel : DialogViewModelBase
+{
+    /// <summary>
+    /// Initializes the message.
+    /// </summary>
+    /// <param name="title">The localized window title.</param>
+    /// <param name="message">The localized message.</param>
+    /// <param name="details">Untranslated diagnostic details, or <see langword="null"/>.</param>
+    public MessageDialogViewModel(string title, string message, string? details)
+    {
+        Title = title;
+        Message = message;
+        Details = details;
+    }
+
+    /// <summary>
+    /// Gets the localized window title.
+    /// </summary>
+    public string Title { get; }
+
+    /// <summary>
+    /// Gets the localized message.
+    /// </summary>
+    public string Message { get; }
+
+    /// <summary>
+    /// Gets the diagnostic details, or <see langword="null"/> when there are none.
+    /// </summary>
+    public string? Details { get; }
+
+    /// <summary>
+    /// Gets whether a details block is shown.
+    /// </summary>
+    public bool HasDetails => !string.IsNullOrEmpty(Details);
+
+    /// <summary>
+    /// Closes the dialog.
+    /// </summary>
+    [RelayCommand]
+    private void Ok() => Close(true);
+}

YKanBan/ViewModels/Dialogs/TagPickerDialogViewModel.cs

@@ -0,0 +1,185 @@
+using System.Collections.ObjectModel;
+using Avalonia.Media;
+using Avalonia.Media.Immutable;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using YKanBan.Models;
+using YKanBan.Storage;
+using YKanBan.Storage.Workspace;
+using YKanBan.ViewModels.Board;
+
+namespace YKanBan.ViewModels.Dialogs;
+
+/// <summary>
+/// The nested "add tag" modal of the card editor: a filterable list of the
+/// existing tags not yet on the card, plus a new-tag entry. Picking a tag closes
+/// the dialog with it as <see cref="PickedTag"/>.
+///
+/// Exception to the edit-dialog unit of work: a tag created here is written to
+/// the tags table immediately (tags are global entities), so it survives even
+/// when the outer card edit is cancelled — only the assignment is discarded.
+/// </summary>
+public sealed partial class TagPickerDialogViewModel : DialogViewModelBase
+{
+    /// <summary>
+    /// Default colors proposed for new tags, cycled by the number of existing tags.
+    /// </summary>
+    private static readonly RgbColor[] Palette =
+    [
+        new(0xd7, 0x3a, 0x4a), new(0x00, 0x75, 0xca), new(0x0e, 0x8a, 0x16), new(0xfb, 0xca, 0x04),
+        new(0x7c, 0x3a, 0xed), new(0xe9, 0x9b, 0x3c), new(0x00, 0x8b, 0x8b), new(0xcf, 0xd3, 0xd7),
+    ];
+
+    private readonly WorkspaceRepository _repository;
+    private readonly IReadOnlyList<TagBadgeViewModel> _available;
+
+    /// <summary>
+    /// Initializes the picker.
+    /// </summary>
+    /// <param name="repository">The workspace repository; new tags are written through it immediately.</param>
+    /// <param name="assignedTagIds">The tags already on the card, which are not offered again.</param>
+    public TagPickerDialogViewModel(WorkspaceRepository repository, IReadOnlyCollection<long> assignedTagIds)
+    {
+        _repository = repository;
+        IReadOnlyList<TagModel> all = repository.GetTags();
+        _available = all
+            .Where(tag => !assignedTagIds.Contains(tag.Id))
+            .Select(tag => new TagBadgeViewModel(tag))
+            .ToList();
+        FilteredTags = new ObservableCollection<TagBadgeViewModel>(_available);
+
+        newColor = Palette[all.Count % Palette.Length].ToString();
+        UpdateColorPreview();
+    }
+
+    /// <summary>
+    /// Gets the tag the user picked or created, once the dialog closed positively.
+    /// </summary>
+    public TagModel? PickedTag { get; private set; }
+
+    /// <summary>
+    /// Gets the available tags matching <see cref="FilterText"/>.
+    /// </summary>
+    public ObservableCollection<TagBadgeViewModel> FilteredTags { get; }
+
+    /// <summary>
+    /// Gets whether no available tag matches the filter.
+    /// </summary>
+    public bool NoMatches => FilteredTags.Count == 0;
+
+    /// <summary>
+    /// Gets or sets the list filter: a case-insensitive substring of the tag name.
+    /// </summary>
+    [ObservableProperty]
+    private string filterText = "";
+
+    /// <summary>
+    /// Gets or sets the name of the tag to create (not trimmed).
+    /// </summary>
+    [ObservableProperty]
+    private string newName = "";
+
+    /// <summary>
+    /// Gets or sets the #RRGGBB color text of the tag to create.
+    /// </summary>
+    [ObservableProperty]
+    private string newColor;
+
+    /// <summary>
+    /// Gets or sets the description of the tag to create.
+    /// </summary>
+    [ObservableProperty]
+    private string newDescription = "";
+
+    /// <summary>
+    /// Gets the preview of <see cref="NewColor"/>, transparent while it is not valid.
+    /// </summary>
+    [ObservableProperty]
+    private IBrush newColorPreview = Brushes.Transparent;
+
+    /// <summary>
+    /// Gets the localized error of the last create attempt, or <see langword="null"/>.
+    /// </summary>
+    [ObservableProperty]
+    [NotifyPropertyChangedFor(nameof(HasError))]
+    private string? errorMessage;
+
+    /// <summary>
+    /// Gets whether an error is shown.
+    /// </summary>
+    public bool HasError => ErrorMessage is not null;
+
+    /// <summary>
+    /// Picks an existing tag and closes.
+    /// </summary>
+    /// <param name="tag">The picked tag.</param>
+    [RelayCommand]
+    private void Pick(TagBadgeViewModel tag)
+    {
+        PickedTag = tag.Tag;
+        Close(true);
+    }
+
+    /// <summary>
+    /// Creates the new tag right away and closes with it picked; constraint
+    /// violations and an invalid color keep the dialog open with an error.
+    /// </summary>
+    [RelayCommand]
+    private void Create()
+    {
+        if (!RgbColor.TryParse(NewColor, out RgbColor color))
+        {
+            ErrorMessage = Resources.Error_Tag_InvalidColor;
+            return;
+        }
+
+        try
+        {
+            PickedTag = _repository.AddTag(NewName, color, NewDescription);
+        }
+        catch (Exception exception)
+        {
+            ErrorMessage = WriteErrorMessages.Describe(exception);
+            return;
+        }
+        Close(true);
+    }
+
+    /// <summary>
+    /// Closes without picking anything.
+    /// </summary>
+    [RelayCommand]
+    private Task CancelAsync() => RequestCancelAsync();
+
+    /// <summary>
+    /// Re-filters the list.
+    /// </summary>
+    /// <param name="value">The new filter text.</param>
+    partial void OnFilterTextChanged(string value)
+    {
+        string folded = SqliteDatabase.Fold(value);
+        FilteredTags.Clear();
+        foreach (TagBadgeViewModel tag in _available)
+        {
+            if (SqliteDatabase.Fold(tag.Name).Contains(folded, StringComparison.Ordinal))
+            {
+                FilteredTags.Add(tag);
+            }
+        }
+        OnPropertyChanged(nameof(NoMatches));
+    }
+
+    /// <summary>
+    /// Refreshes the color preview.
+    /// </summary>
+    /// <param name="value">The new color text.</param>
+    partial void OnNewColorChanged(string value) => UpdateColorPreview();
+
+    /// <summary>
+    /// Recomputes <see cref="NewColorPreview"/> from <see cref="NewColor"/>.
+    /// </summary>
+    private void UpdateColorPreview() =>
+        NewColorPreview = RgbColor.TryParse(NewColor, out RgbColor color)
+            ? new ImmutableSolidColorBrush(Color.FromRgb(color.R, color.G, color.B))
+            : Brushes.Transparent;
+}

YKanBan/ViewModels/Dialogs/WriteErrorMessages.cs

@@ -0,0 +1,33 @@
+using System.Globalization;
+using YKanBan.Storage;
+
+namespace YKanBan.ViewModels.Dialogs;
+
+/// <summary>
+/// Maps a failed dialog write to the localized message shown inside the dialog
+/// that attempted it (the dialog stays open and keeps the user's input). The
+/// typed schema-constraint exceptions get dedicated messages; anything else
+/// gets a generic one carrying the untranslated exception message.
+/// </summary>
+public static class WriteErrorMessages
+{
+    /// <summary>
+    /// Returns the localized message for a failed write.
+    /// </summary>
+    /// <param name="exception">The exception thrown by the write.</param>
+    /// <returns>The localized message.</returns>
+    public static string Describe(Exception exception)
+    {
+        CultureInfo? culture = Resources.Culture;
+        return exception switch
+        {
+            NameRequiredException { Entity: NamedEntity.Column } => Resources.Error_Column_TitleRequired,
+            NameRequiredException { Entity: NamedEntity.Tag } => Resources.Error_Tag_NameRequired,
+            DuplicateNameException { Entity: NamedEntity.Column } duplicate =>
+                string.Format(culture, Resources.Error_Column_Duplicate, duplicate.Name),
+            DuplicateNameException { Entity: NamedEntity.Tag } duplicate =>
+                string.Format(culture, Resources.Error_Tag_Duplicate, duplicate.Name),
+            _ => string.Format(culture, Resources.Error_SaveFailed, exception.Message),
+        };
+    }
+}

YKanBan/ViewModels/MainWindowViewModel.cs

@@ -4,6 +4,7 @@ using YKanBan.Launching;
 using YKanBan.Services;
 using YKanBan.Storage;
 using YKanBan.Storage.Workspace;
+using YKanBan.ViewModels.Dialogs;
 using YKanBan.ViewModels.ErrorPages;
 
 namespace YKanBan.ViewModels;
@@ -16,15 +17,18 @@ namespace YKanBan.ViewModels;
 public sealed partial class MainWindowViewModel : ViewModelBase
 {
     private readonly AppServices _services;
+    private readonly IDialogService _dialogs;
 
     /// <summary>
     /// Initializes the view model and resolves the workspace state.
     /// </summary>
     /// <param name="services">The process composition root.</param>
+    /// <param name="dialogs">The modal dialog service used by the board.</param>
     /// <param name="arguments">The command-line arguments (excluding the executable).</param>
-    public MainWindowViewModel(AppServices services, IReadOnlyList<string> arguments)
+    public MainWindowViewModel(AppServices services, IDialogService dialogs, IReadOnlyList<string> arguments)
     {
         _services = services;
+        _dialogs = dialogs;
         ShowSettings = new Interaction<Unit>();
         ShowAbout = new Interaction<Unit>();
         InitializeWorkspace = new Interaction<Unit>();
@@ -104,7 +108,7 @@ public sealed partial class MainWindowViewModel : ViewModelBase
     private void OpenWorkspace(string folderPath)
     {
         WorkspaceSession session = _services.OpenWorkspace(folderPath);
-        Content = new WorkspaceViewModel(session, ShowSettings, ShowAbout);
+        Content = new WorkspaceViewModel(session, ShowSettings, ShowAbout, _services.Config, _dialogs);
         Title = $"{new DirectoryInfo(folderPath).Name} - {Resources.App_Name}";
     }
 

YKanBan/ViewModels/WorkspaceViewModel.cs

@@ -1,5 +1,8 @@
 using CommunityToolkit.Mvvm.Input;
+using YKanBan.Storage.AppData;
 using YKanBan.Storage.Workspace;
+using YKanBan.ViewModels.Board;
+using YKanBan.ViewModels.Dialogs;
 
 namespace YKanBan.ViewModels;
 
@@ -19,16 +22,21 @@ public sealed partial class WorkspaceViewModel : ViewModelBase
     /// <param name="session">The open workspace session.</param>
     /// <param name="showSettings">The interaction that opens the settings dialog.</param>
     /// <param name="showAbout">The interaction that opens the about dialog.</param>
+    /// <param name="config">The live application configuration.</param>
+    /// <param name="dialogs">The modal dialog service of the board.</param>
     public WorkspaceViewModel(
         WorkspaceSession session,
         Interaction<Unit> showSettings,
-        Interaction<Unit> showAbout)
+        Interaction<Unit> showAbout,
+        AppConfig config,
+        IDialogService dialogs)
     {
         Session = session;
         WorkspaceName = new DirectoryInfo(session.FolderPath).Name;
         WorkspacePath = session.FolderPath;
         _showSettings = showSettings;
         _showAbout = showAbout;
+        Board = new BoardViewModel(session.Repository, config, dialogs);
     }
 
     /// <summary>
@@ -46,6 +54,11 @@ public sealed partial class WorkspaceViewModel : ViewModelBase
     /// </summary>
     public string WorkspacePath { get; }
 
+    /// <summary>
+    /// Gets the board tab.
+    /// </summary>
+    public BoardViewModel Board { get; }
+
     /// <summary>
     /// Opens the settings dialog through the registered interaction.
     /// </summary>

YKanBan/Views/Board/BoardView.axaml

@@ -0,0 +1,176 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:vm="using:YKanBan.ViewModels.Board"
+             xmlns:vmBase="using:YKanBan.ViewModels"
+             xmlns:lang="using:YKanBan"
+             xmlns:icons="using:IconPacks.Avalonia.Lucide"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             mc:Ignorable="d" d:DesignWidth="1000" d:DesignHeight="600"
+             x:Class="YKanBan.Views.Board.BoardView"
+             x:DataType="vm:BoardViewModel"
+             x:Name="Root">
+
+    <Grid RowDefinitions="Auto,*" RowSpacing="8">
+
+        <!-- Toolbar. The search bar joins on the left with the search feature. -->
+        <StackPanel Grid.Row="0" Orientation="Horizontal" HorizontalAlignment="Right" Spacing="8">
+            <Button Command="{Binding AddColumnCommand}">
+                <StackPanel Orientation="Horizontal" Spacing="6">
+                    <icons:PackIconLucide Kind="Plus" Width="16" Height="16"/>
+                    <TextBlock Text="{x:Static lang:Resources.Board_AddColumn}"/>
+                </StackPanel>
+            </Button>
+            <Button Command="{Binding AdjustColumnWidthCommand}">
+                <StackPanel Orientation="Horizontal" Spacing="6">
+                    <icons:PackIconLucide Kind="MoveHorizontal" Width="16" Height="16"/>
+                    <TextBlock Text="{x:Static lang:Resources.Board_ColumnWidth}"/>
+                </StackPanel>
+            </Button>
+            <ComboBox ItemsSource="{Binding SortOptions}" SelectedItem="{Binding SelectedSort}" MinWidth="160"
+                      ToolTip.Tip="{x:Static lang:Resources.Board_Sort}">
+                <ComboBox.ItemTemplate>
+                    <DataTemplate x:DataType="vmBase:OptionItem">
+                        <TextBlock Text="{Binding Label}"/>
+                    </DataTemplate>
+                </ComboBox.ItemTemplate>
+            </ComboBox>
+        </StackPanel>
+
+        <!-- Columns left to right; each column scrolls its own virtualized card list. -->
+        <ScrollViewer Grid.Row="1" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Disabled">
+            <ItemsControl ItemsSource="{Binding Columns}">
+                <ItemsControl.ItemsPanel>
+                    <ItemsPanelTemplate>
+                        <StackPanel Orientation="Horizontal" Spacing="12"/>
+                    </ItemsPanelTemplate>
+                </ItemsControl.ItemsPanel>
+                <ItemsControl.ItemTemplate>
+                    <DataTemplate x:DataType="vm:ColumnViewModel">
+                        <Border Width="{Binding #Root.((vm:BoardViewModel)DataContext).ColumnWidth}"
+                                Background="{DynamicResource ColumnBackgroundBrush}"
+                                BorderBrush="{DynamicResource PanelBorderBrush}" BorderThickness="1"
+                                CornerRadius="6" Padding="8">
+                            <Grid RowDefinitions="Auto,*" RowSpacing="8">
+
+                                <!-- Column header: title, count, description, new card, edit/delete menu. -->
+                                <Grid Grid.Row="0" ColumnDefinitions="*,Auto,Auto">
+                                    <StackPanel Grid.Column="0" Spacing="2" VerticalAlignment="Center">
+                                        <StackPanel Orientation="Horizontal" Spacing="6">
+                                            <TextBlock Text="{Binding Title}" FontWeight="SemiBold"
+                                                       TextTrimming="CharacterEllipsis"/>
+                                            <TextBlock Text="{Binding CardCount}" Opacity="0.6"/>
+                                        </StackPanel>
+                                        <TextBlock Text="{Binding Description}" IsVisible="{Binding HasDescription}"
+                                                   FontSize="12" Opacity="0.7" TextWrapping="Wrap" MaxLines="3"
+                                                   TextTrimming="CharacterEllipsis"/>
+                                    </StackPanel>
+                                    <Button Grid.Column="1" Command="{Binding AddCardCommand}" VerticalAlignment="Top"
+                                            Theme="{StaticResource BorderlessButton}" Padding="4"
+                                            ToolTip.Tip="{x:Static lang:Resources.Board_AddCard}">
+                                        <icons:PackIconLucide Kind="Plus" Width="16" Height="16"/>
+                                    </Button>
+                                    <Button Grid.Column="2" VerticalAlignment="Top"
+                                            Theme="{StaticResource BorderlessButton}" Padding="4"
+                                            ToolTip.Tip="{x:Static lang:Resources.Menu_More}">
+                                        <icons:PackIconLucide Kind="Ellipsis" Width="16" Height="16"/>
+                                        <Button.Flyout>
+                                            <MenuFlyout Placement="BottomEdgeAlignedRight">
+                                                <MenuItem Header="{x:Static lang:Resources.Menu_Edit}" Command="{Binding EditCommand}"/>
+                                                <MenuItem Header="{x:Static lang:Resources.Menu_Delete}" Command="{Binding DeleteCommand}"/>
+                                            </MenuFlyout>
+                                        </Button.Flyout>
+                                    </Button>
+                                </Grid>
+
+                                <!-- Cards top to bottom, UI-virtualized for large columns. -->
+                                <ItemsControl Grid.Row="1" ItemsSource="{Binding Cards}">
+                                    <ItemsControl.Template>
+                                        <ControlTemplate>
+                                            <ScrollViewer HorizontalScrollBarVisibility="Disabled"
+                                                          VerticalScrollBarVisibility="Auto">
+                                                <ItemsPresenter Name="PART_ItemsPresenter"
+                                                                ItemsPanel="{TemplateBinding ItemsPanel}"/>
+                                            </ScrollViewer>
+                                        </ControlTemplate>
+                                    </ItemsControl.Template>
+                                    <ItemsControl.ItemsPanel>
+                                        <ItemsPanelTemplate>
+                                            <VirtualizingStackPanel/>
+                                        </ItemsPanelTemplate>
+                                    </ItemsControl.ItemsPanel>
+                                    <ItemsControl.ItemTemplate>
+                                        <DataTemplate x:DataType="vm:CardViewModel">
+                                            <Panel Margin="0,0,0,8">
+                                                <!-- The card body opens the editor. -->
+                                                <Button Command="{Binding EditCommand}"
+                                                        HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch"
+                                                        VerticalContentAlignment="Top"
+                                                        Background="{DynamicResource CardBackgroundBrush}"
+                                                        Foreground="{DynamicResource CardForegroundBrush}"
+                                                        FontWeight="Normal"
+                                                        BorderBrush="{DynamicResource PanelBorderBrush}" BorderThickness="1"
+                                                        CornerRadius="4" Padding="10,8" Cursor="Hand">
+                                                    <StackPanel Spacing="6">
+                                                        <TextBlock TextWrapping="Wrap" Margin="0,0,24,0">
+                                                            <Run Text="{Binding IdText}" FontWeight="Bold"/><Run Text="{Binding TitleSuffix}"/>
+                                                        </TextBlock>
+                                                        <TextBlock Text="{Binding Excerpt}" IsVisible="{Binding HasExcerpt}"
+                                                                   TextWrapping="Wrap" FontSize="12" Opacity="0.8"
+                                                                   FontWeight="Normal"/>
+                                                        <ItemsControl ItemsSource="{Binding Tags}" IsVisible="{Binding HasTags}">
+                                                            <ItemsControl.ItemsPanel>
+                                                                <ItemsPanelTemplate>
+                                                                    <WrapPanel/>
+                                                                </ItemsPanelTemplate>
+                                                            </ItemsControl.ItemsPanel>
+                                                            <ItemsControl.ItemTemplate>
+                                                                <DataTemplate x:DataType="vm:TagBadgeViewModel">
+                                                                    <Border Background="{Binding Background}" CornerRadius="10"
+                                                                            Padding="8,1" Margin="0,0,4,4">
+                                                                        <TextBlock Text="{Binding Name}" Foreground="{Binding Foreground}"
+                                                                                   FontSize="11" FontWeight="Normal"/>
+                                                                    </Border>
+                                                                </DataTemplate>
+                                                            </ItemsControl.ItemTemplate>
+                                                        </ItemsControl>
+                                                    </StackPanel>
+                                                </Button>
+
+                                                <!-- Card menu: delete, move to column. -->
+                                                <Button HorizontalAlignment="Right" VerticalAlignment="Top" Margin="0,4,4,0"
+                                                        Theme="{StaticResource BorderlessButton}" Padding="4"
+                                                        ToolTip.Tip="{x:Static lang:Resources.Menu_More}">
+                                                    <icons:PackIconLucide Kind="Ellipsis" Width="14" Height="14"/>
+                                                    <Button.Flyout>
+                                                        <MenuFlyout Placement="BottomEdgeAlignedRight">
+                                                            <MenuItem Header="{x:Static lang:Resources.Menu_Delete}"
+                                                                      Command="{Binding DeleteCommand}"/>
+                                                            <MenuItem Header="{x:Static lang:Resources.Menu_MoveTo}"
+                                                                      ItemsSource="{Binding MoveTargets}">
+                                                                <MenuItem.ItemContainerTheme>
+                                                                    <ControlTheme TargetType="MenuItem"
+                                                                                  BasedOn="{StaticResource {x:Type MenuItem}}"
+                                                                                  x:DataType="vm:MoveTargetViewModel">
+                                                                        <Setter Property="Header" Value="{Binding Title}"/>
+                                                                        <Setter Property="Command" Value="{Binding Command}"/>
+                                                                        <Setter Property="CommandParameter" Value="{Binding ColumnId}"/>
+                                                                        <Setter Property="IsEnabled" Value="{Binding IsEnabled}"/>
+                                                                    </ControlTheme>
+                                                                </MenuItem.ItemContainerTheme>
+                                                            </MenuItem>
+                                                        </MenuFlyout>
+                                                    </Button.Flyout>
+                                                </Button>
+                                            </Panel>
+                                        </DataTemplate>
+                                    </ItemsControl.ItemTemplate>
+                                </ItemsControl>
+                            </Grid>
+                        </Border>
+                    </DataTemplate>
+                </ItemsControl.ItemTemplate>
+            </ItemsControl>
+        </ScrollViewer>
+    </Grid>
+</UserControl>

YKanBan/Views/Board/BoardView.axaml.cs

@@ -0,0 +1,11 @@
+using Avalonia.Controls;
+
+namespace YKanBan.Views.Board;
+
+public partial class BoardView : UserControl
+{
+    public BoardView()
+    {
+        InitializeComponent();
+    }
+}

YKanBan/Views/DialogService.cs

@@ -0,0 +1,68 @@
+using Avalonia.Controls;
+using YKanBan.ViewModels.Dialogs;
+using YKanBan.Views.Dialogs;
+
+namespace YKanBan.Views;
+
+/// <summary>
+/// Shows dialog view models as modal windows. A <c>XxxDialogViewModel</c> in
+/// <c>YKanBan.ViewModels.Dialogs</c> is shown by the <c>XxxDialog</c> window in
+/// <c>YKanBan.Views.Dialogs</c>. Each dialog is owned by the top-most open
+/// window, so nested modals (the tag picker inside the card editor, a discard
+/// confirmation above it) center on their direct parent.
+/// </summary>
+public sealed class DialogService : IDialogService
+{
+    private readonly Stack<Window> _owners = new();
+
+    /// <summary>
+    /// Sets the main window as the bottom of the owner stack.
+    /// </summary>
+    /// <param name="mainWindow">The main window.</param>
+    public void Attach(Window mainWindow)
+    {
+        _owners.Clear();
+        _owners.Push(mainWindow);
+    }
+
+    /// <inheritdoc/>
+    public async Task<bool> ShowAsync(DialogViewModelBase dialog)
+    {
+        if (!_owners.TryPeek(out Window? owner))
+        {
+            throw new InvalidOperationException("The dialog service has no main window attached.");
+        }
+
+        DialogWindow window = CreateWindow(dialog);
+        window.DataContext = dialog;
+
+        // FlowDirection does not inherit across windows, so every modal gets it explicitly.
+        window.FlowDirection = App.UiFlowDirection;
+
+        _owners.Push(window);
+        try
+        {
+            return await window.ShowDialog<bool>(owner);
+        }
+        finally
+        {
+            _owners.Pop();
+        }
+    }
+
+    /// <summary>
+    /// Creates the window class matching a dialog view model by naming convention.
+    /// </summary>
+    /// <param name="dialog">The dialog view model.</param>
+    /// <returns>The window.</returns>
+    private static DialogWindow CreateWindow(DialogViewModelBase dialog)
+    {
+        string viewName = typeof(ConfirmDialog).Namespace + "." + dialog.GetType().Name.Replace("ViewModel", "", StringComparison.Ordinal);
+        Type? viewType = typeof(DialogService).Assembly.GetType(viewName);
+        if (viewType is null || !typeof(DialogWindow).IsAssignableFrom(viewType))
+        {
+            throw new InvalidOperationException($"No dialog window '{viewName}' for {dialog.GetType().Name}.");
+        }
+        return (DialogWindow)Activator.CreateInstance(viewType)!;
+    }
+}

YKanBan/Views/Dialogs/CardEditorDialog.axaml

@@ -0,0 +1,81 @@
+<dialogs:DialogWindow xmlns="https://github.com/avaloniaui"
+                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                      xmlns:vm="using:YKanBan.ViewModels.Dialogs"
+                      xmlns:board="using:YKanBan.ViewModels.Board"
+                      xmlns:dialogs="using:YKanBan.Views.Dialogs"
+                      xmlns:lang="using:YKanBan"
+                      xmlns:icons="using:IconPacks.Avalonia.Lucide"
+                      x:Class="YKanBan.Views.Dialogs.CardEditorDialog"
+                      x:DataType="vm:CardEditorDialogViewModel"
+                      x:Name="Root"
+                      Title="{Binding Title}"
+                      SizeToContent="Manual" CanResize="True"
+                      Width="600" Height="560" MinWidth="440" MinHeight="420"
+                      WindowStartupLocation="CenterOwner">
+
+    <Grid Margin="20" RowDefinitions="Auto,Auto,*,Auto,Auto,Auto,Auto" RowSpacing="12">
+
+        <!-- Read-only id and timestamps; not shown while the card does not exist yet. -->
+        <WrapPanel Grid.Row="0" IsVisible="{Binding IsExisting}" Orientation="Horizontal">
+            <TextBlock Text="{Binding IdText}" FontWeight="Bold" Margin="0,0,16,0"/>
+            <TextBlock Text="{Binding CreatedText}" Opacity="0.7" Margin="0,0,16,0"/>
+            <TextBlock Text="{Binding ModifiedText}" Opacity="0.7"/>
+        </WrapPanel>
+
+        <StackPanel Grid.Row="1" Spacing="4">
+            <TextBlock Text="{x:Static lang:Resources.Field_Title}"/>
+            <TextBox Text="{Binding CardTitle}" Watermark="{x:Static lang:Resources.CardEditor_TitleWatermark}"/>
+        </StackPanel>
+
+        <Grid Grid.Row="2" RowDefinitions="Auto,*" RowSpacing="4">
+            <TextBlock Grid.Row="0" Text="{x:Static lang:Resources.Field_Content}"/>
+            <TextBox Grid.Row="1" Text="{Binding Content}" AcceptsReturn="True" TextWrapping="Wrap"
+                     VerticalAlignment="Stretch"
+                     VerticalContentAlignment="Top"/>
+        </Grid>
+
+        <StackPanel Grid.Row="3" Spacing="6">
+            <TextBlock Text="{x:Static lang:Resources.Field_Tags}"/>
+            <ItemsControl ItemsSource="{Binding Tags}">
+                <ItemsControl.ItemsPanel>
+                    <ItemsPanelTemplate>
+                        <WrapPanel/>
+                    </ItemsPanelTemplate>
+                </ItemsControl.ItemsPanel>
+                <ItemsControl.ItemTemplate>
+                    <DataTemplate x:DataType="board:TagBadgeViewModel">
+                        <Border Background="{Binding Background}" CornerRadius="10" Padding="8,2,4,2"
+                                Margin="0,0,6,6" ToolTip.Tip="{Binding Description}">
+                            <StackPanel Orientation="Horizontal" Spacing="2">
+                                <TextBlock Text="{Binding Name}" Foreground="{Binding Foreground}"
+                                           VerticalAlignment="Center" FontSize="12"/>
+                                <Button Command="{Binding #Root.((vm:CardEditorDialogViewModel)DataContext).RemoveTagCommand}"
+                                        CommandParameter="{Binding}"
+                                        Theme="{StaticResource BorderlessButton}" Padding="2"
+                                        ToolTip.Tip="{x:Static lang:Resources.CardEditor_RemoveTag}">
+                                    <icons:PackIconLucide Kind="X" Width="12" Height="12" Foreground="{Binding Foreground}"/>
+                                </Button>
+                            </StackPanel>
+                        </Border>
+                    </DataTemplate>
+                </ItemsControl.ItemTemplate>
+            </ItemsControl>
+            <Button Command="{Binding AddTagCommand}" HorizontalAlignment="Left">
+                <StackPanel Orientation="Horizontal" Spacing="6">
+                    <icons:PackIconLucide Kind="Plus" Width="14" Height="14"/>
+                    <TextBlock Text="{x:Static lang:Resources.CardEditor_AddTag}"/>
+                </StackPanel>
+            </Button>
+        </StackPanel>
+
+        <TextBlock Grid.Row="4" Text="{Binding ErrorMessage}" IsVisible="{Binding HasError}"
+                   Foreground="{DynamicResource ErrorTextBrush}" TextWrapping="Wrap"/>
+
+        <StackPanel Grid.Row="6" Orientation="Horizontal" HorizontalAlignment="Right" Spacing="8">
+            <Button Content="{x:Static lang:Resources.Common_Ok}" Command="{Binding ConfirmCommand}"
+                    Classes="Primary" MinWidth="90"/>
+            <Button Content="{x:Static lang:Resources.Common_Cancel}" Command="{Binding CancelCommand}"
+                    MinWidth="90" IsCancel="True"/>
+        </StackPanel>
+    </Grid>
+</dialogs:DialogWindow>

YKanBan/Views/Dialogs/CardEditorDialog.axaml.cs

@@ -0,0 +1,9 @@
+namespace YKanBan.Views.Dialogs;
+
+public partial class CardEditorDialog : DialogWindow
+{
+    public CardEditorDialog()
+    {
+        InitializeComponent();
+    }
+}

YKanBan/Views/Dialogs/ColumnEditorDialog.axaml

@@ -0,0 +1,33 @@
+<dialogs:DialogWindow xmlns="https://github.com/avaloniaui"
+                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                      xmlns:vm="using:YKanBan.ViewModels.Dialogs"
+                      xmlns:dialogs="using:YKanBan.Views.Dialogs"
+                      xmlns:lang="using:YKanBan"
+                      x:Class="YKanBan.Views.Dialogs.ColumnEditorDialog"
+                      x:DataType="vm:ColumnEditorDialogViewModel"
+                      Title="{Binding Title}"
+                      Width="440"
+                      WindowStartupLocation="CenterOwner">
+
+    <StackPanel Margin="20" Spacing="14">
+        <StackPanel Spacing="4">
+            <TextBlock Text="{x:Static lang:Resources.Field_Title}"/>
+            <TextBox Text="{Binding ColumnTitle}"/>
+        </StackPanel>
+        <StackPanel Spacing="4">
+            <TextBlock Text="{x:Static lang:Resources.Field_Description}"/>
+            <TextBox Text="{Binding Description}" AcceptsReturn="True" TextWrapping="Wrap"
+                     MinHeight="72" MaxHeight="200"/>
+        </StackPanel>
+
+        <TextBlock Text="{Binding ErrorMessage}" IsVisible="{Binding HasError}"
+                   Foreground="{DynamicResource ErrorTextBrush}" TextWrapping="Wrap"/>
+
+        <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Spacing="8">
+            <Button Content="{x:Static lang:Resources.Common_Ok}" Command="{Binding ConfirmCommand}"
+                    Classes="Primary" MinWidth="90" IsDefault="True"/>
+            <Button Content="{x:Static lang:Resources.Common_Cancel}" Command="{Binding CancelCommand}"
+                    MinWidth="90" IsCancel="True"/>
+        </StackPanel>
+    </StackPanel>
+</dialogs:DialogWindow>

YKanBan/Views/Dialogs/ColumnEditorDialog.axaml.cs

@@ -0,0 +1,9 @@
+namespace YKanBan.Views.Dialogs;
+
+public partial class ColumnEditorDialog : DialogWindow
+{
+    public ColumnEditorDialog()
+    {
+        InitializeComponent();
+    }
+}

YKanBan/Views/Dialogs/ColumnWidthDialog.axaml

@@ -0,0 +1,30 @@
+<dialogs:DialogWindow xmlns="https://github.com/avaloniaui"
+                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                      xmlns:vm="using:YKanBan.ViewModels.Dialogs"
+                      xmlns:dialogs="using:YKanBan.Views.Dialogs"
+                      xmlns:lang="using:YKanBan"
+                      x:Class="YKanBan.Views.Dialogs.ColumnWidthDialog"
+                      x:DataType="vm:ColumnWidthDialogViewModel"
+                      Title="{x:Static lang:Resources.ColumnWidth_Title}"
+                      Width="360"
+                      WindowStartupLocation="CenterOwner">
+
+    <StackPanel Margin="20" Spacing="10">
+        <RadioButton GroupName="Width" Content="{Binding NarrowLabel}" IsChecked="{Binding IsNarrow}"/>
+        <RadioButton GroupName="Width" Content="{Binding StandardLabel}" IsChecked="{Binding IsStandard}"/>
+        <RadioButton GroupName="Width" Content="{Binding WideLabel}" IsChecked="{Binding IsWide}"/>
+        <RadioButton GroupName="Width" Content="{x:Static lang:Resources.ColumnWidth_Custom}" IsChecked="{Binding IsCustom}"/>
+        <StackPanel Orientation="Horizontal" Spacing="8" Margin="28,0,0,0" IsEnabled="{Binding IsCustom}">
+            <NumericUpDown Value="{Binding CustomPixels}" Minimum="{Binding MinPixels}" Maximum="{Binding MaxPixels}"
+                           Increment="10" FormatString="0" Width="140"/>
+            <TextBlock Text="{Binding RangeLabel}" VerticalAlignment="Center" Opacity="0.7"/>
+        </StackPanel>
+
+        <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Spacing="8" Margin="0,8,0,0">
+            <Button Content="{x:Static lang:Resources.Common_Ok}" Command="{Binding ConfirmCommand}"
+                    Classes="Primary" MinWidth="90" IsDefault="True"/>
+            <Button Content="{x:Static lang:Resources.Common_Cancel}" Command="{Binding CancelCommand}"
+                    MinWidth="90" IsCancel="True"/>
+        </StackPanel>
+    </StackPanel>
+</dialogs:DialogWindow>

YKanBan/Views/Dialogs/ColumnWidthDialog.axaml.cs

@@ -0,0 +1,9 @@
+namespace YKanBan.Views.Dialogs;
+
+public partial class ColumnWidthDialog : DialogWindow
+{
+    public ColumnWidthDialog()
+    {
+        InitializeComponent();
+    }
+}

YKanBan/Views/Dialogs/ConfirmDialog.axaml

@@ -0,0 +1,21 @@
+<dialogs:DialogWindow xmlns="https://github.com/avaloniaui"
+                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                      xmlns:vm="using:YKanBan.ViewModels.Dialogs"
+                      xmlns:dialogs="using:YKanBan.Views.Dialogs"
+                      xmlns:lang="using:YKanBan"
+                      x:Class="YKanBan.Views.Dialogs.ConfirmDialog"
+                      x:DataType="vm:ConfirmDialogViewModel"
+                      Title="{x:Static lang:Resources.Confirm_Title}"
+                      MinWidth="360" MaxWidth="520"
+                      WindowStartupLocation="CenterOwner">
+
+    <StackPanel Margin="20" Spacing="16">
+        <TextBlock Text="{Binding Message}" TextWrapping="Wrap"/>
+        <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Spacing="8">
+            <Button Content="{Binding ConfirmLabel}" Command="{Binding ConfirmCommand}"
+                    Classes="Danger" MinWidth="90" IsDefault="True"/>
+            <Button Content="{x:Static lang:Resources.Common_Cancel}" Command="{Binding CancelCommand}"
+                    MinWidth="90" IsCancel="True"/>
+        </StackPanel>
+    </StackPanel>
+</dialogs:DialogWindow>

YKanBan/Views/Dialogs/ConfirmDialog.axaml.cs

@@ -0,0 +1,9 @@
+namespace YKanBan.Views.Dialogs;
+
+public partial class ConfirmDialog : DialogWindow
+{
+    public ConfirmDialog()
+    {
+        InitializeComponent();
+    }
+}

YKanBan/Views/Dialogs/DialogWindow.cs

@@ -0,0 +1,86 @@
+using Avalonia.Controls;
+using YKanBan.ViewModels.Dialogs;
+
+namespace YKanBan.Views.Dialogs;
+
+/// <summary>
+/// Base class of the windows shown by <see cref="DialogService"/>: a standard
+/// Avalonia window centered on its owner that closes when its
+/// <see cref="DialogViewModelBase"/> asks, with the result as the dialog result.
+/// The title bar close button is routed to the view model's cancel path, so a
+/// dialog with unsaved changes can ask before discarding them.
+/// </summary>
+public class DialogWindow : Window
+{
+    private DialogViewModelBase? _viewModel;
+    private bool _closeRequested;
+    private bool _cancelPending;
+
+    public DialogWindow()
+    {
+        WindowStartupLocation = WindowStartupLocation.CenterOwner;
+        ShowInTaskbar = false;
+        CanResize = false;
+        SizeToContent = SizeToContent.WidthAndHeight;
+    }
+
+    /// <summary>Styles the subclasses as plain windows.</summary>
+    protected override Type StyleKeyOverride => typeof(Window);
+
+    protected override void OnDataContextChanged(EventArgs e)
+    {
+        base.OnDataContextChanged(e);
+        if (_viewModel is not null)
+        {
+            _viewModel.CloseRequested -= OnCloseRequested;
+        }
+        _viewModel = DataContext as DialogViewModelBase;
+        if (_viewModel is not null)
+        {
+            _viewModel.CloseRequested += OnCloseRequested;
+        }
+    }
+
+    protected override async void OnClosing(WindowClosingEventArgs e)
+    {
+        base.OnClosing(e);
+
+        // Only the user's close button goes through the view model; shutdown and
+        // owner closing are let through.
+        if (_closeRequested || _viewModel is null || e.CloseReason != WindowCloseReason.WindowClosing)
+        {
+            return;
+        }
+
+        e.Cancel = true;
+        if (_cancelPending)
+        {
+            return;
+        }
+
+        _cancelPending = true;
+        try
+        {
+            await _viewModel.RequestCancelAsync();
+        }
+        finally
+        {
+            _cancelPending = false;
+        }
+    }
+
+    protected override void OnClosed(EventArgs e)
+    {
+        base.OnClosed(e);
+        if (_viewModel is not null)
+        {
+            _viewModel.CloseRequested -= OnCloseRequested;
+        }
+    }
+
+    private void OnCloseRequested(object? sender, bool result)
+    {
+        _closeRequested = true;
+        Close(result);
+    }
+}

YKanBan/Views/Dialogs/MessageDialog.axaml

@@ -0,0 +1,19 @@
+<dialogs:DialogWindow xmlns="https://github.com/avaloniaui"
+                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                      xmlns:vm="using:YKanBan.ViewModels.Dialogs"
+                      xmlns:dialogs="using:YKanBan.Views.Dialogs"
+                      xmlns:lang="using:YKanBan"
+                      x:Class="YKanBan.Views.Dialogs.MessageDialog"
+                      x:DataType="vm:MessageDialogViewModel"
+                      Title="{Binding Title}"
+                      MinWidth="360" MaxWidth="560"
+                      WindowStartupLocation="CenterOwner">
+
+    <StackPanel Margin="20" Spacing="12">
+        <TextBlock Text="{Binding Message}" TextWrapping="Wrap"/>
+        <SelectableTextBlock Text="{Binding Details}" IsVisible="{Binding HasDetails}"
+                             TextWrapping="Wrap" FontSize="12" Opacity="0.7"/>
+        <Button Content="{x:Static lang:Resources.Common_Ok}" Command="{Binding OkCommand}"
+                HorizontalAlignment="Right" MinWidth="90" IsDefault="True" IsCancel="True"/>
+    </StackPanel>
+</dialogs:DialogWindow>

YKanBan/Views/Dialogs/MessageDialog.axaml.cs

@@ -0,0 +1,9 @@
+namespace YKanBan.Views.Dialogs;
+
+public partial class MessageDialog : DialogWindow
+{
+    public MessageDialog()
+    {
+        InitializeComponent();
+    }
+}

YKanBan/Views/Dialogs/TagPickerDialog.axaml

@@ -0,0 +1,76 @@
+<dialogs:DialogWindow xmlns="https://github.com/avaloniaui"
+                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                      xmlns:vm="using:YKanBan.ViewModels.Dialogs"
+                      xmlns:board="using:YKanBan.ViewModels.Board"
+                      xmlns:dialogs="using:YKanBan.Views.Dialogs"
+                      xmlns:lang="using:YKanBan"
+                      xmlns:icons="using:IconPacks.Avalonia.Lucide"
+                      x:Class="YKanBan.Views.Dialogs.TagPickerDialog"
+                      x:DataType="vm:TagPickerDialogViewModel"
+                      x:Name="Root"
+                      Title="{x:Static lang:Resources.TagPicker_Title}"
+                      SizeToContent="Manual" CanResize="True"
+                      Width="460" Height="560" MinWidth="380" MinHeight="440"
+                      WindowStartupLocation="CenterOwner">
+
+    <Grid Margin="20" RowDefinitions="Auto,*,Auto,Auto" RowSpacing="12">
+
+        <TextBox Grid.Row="0" Text="{Binding FilterText}" Watermark="{x:Static lang:Resources.TagPicker_FilterWatermark}">
+            <TextBox.InnerLeftContent>
+                <icons:PackIconLucide Kind="Search" Width="14" Height="14" Margin="8,0,0,0" Opacity="0.6"
+                                      VerticalAlignment="Center"/>
+            </TextBox.InnerLeftContent>
+        </TextBox>
+
+        <!-- Existing tags not yet on the card; one click picks a tag. -->
+        <Border Grid.Row="1" BorderBrush="{DynamicResource PanelBorderBrush}" BorderThickness="1" CornerRadius="4">
+            <Panel>
+                <ListBox ItemsSource="{Binding FilteredTags}" Background="Transparent">
+                    <ListBox.ItemTemplate>
+                        <DataTemplate x:DataType="board:TagBadgeViewModel">
+                            <Button Command="{Binding #Root.((vm:TagPickerDialogViewModel)DataContext).PickCommand}"
+                                    CommandParameter="{Binding}"
+                                    Theme="{StaticResource BorderlessButton}"
+                                    HorizontalAlignment="Stretch" HorizontalContentAlignment="Left" Padding="4">
+                                <StackPanel Orientation="Horizontal" Spacing="10">
+                                    <Border Background="{Binding Background}" CornerRadius="10" Padding="8,2">
+                                        <TextBlock Text="{Binding Name}" Foreground="{Binding Foreground}" FontSize="12"/>
+                                    </Border>
+                                    <TextBlock Text="{Binding Description}" Opacity="0.7" VerticalAlignment="Center"
+                                               TextTrimming="CharacterEllipsis"/>
+                                </StackPanel>
+                            </Button>
+                        </DataTemplate>
+                    </ListBox.ItemTemplate>
+                </ListBox>
+                <TextBlock Text="{x:Static lang:Resources.TagPicker_NoMatches}" IsVisible="{Binding NoMatches}"
+                           HorizontalAlignment="Center" VerticalAlignment="Center" Opacity="0.6"/>
+            </Panel>
+        </Border>
+
+        <!-- New tag: written to the tags table right away, even if the card edit is cancelled. -->
+        <StackPanel Grid.Row="2" Spacing="8">
+            <TextBlock Text="{x:Static lang:Resources.TagPicker_NewTag}" FontWeight="SemiBold"/>
+            <Grid ColumnDefinitions="*,Auto,Auto" ColumnSpacing="8">
+                <TextBox Grid.Column="0" Text="{Binding NewName}" Watermark="{x:Static lang:Resources.Field_Name}"/>
+                <TextBox Grid.Column="1" Text="{Binding NewColor}" Width="100"
+                         Watermark="{x:Static lang:Resources.Field_Color}"/>
+                <Border Grid.Column="2" Width="28" Height="28" CornerRadius="4"
+                        Background="{Binding NewColorPreview}"
+                        BorderBrush="{DynamicResource PanelBorderBrush}" BorderThickness="1"/>
+            </Grid>
+            <TextBox Text="{Binding NewDescription}" Watermark="{x:Static lang:Resources.Field_Description}"/>
+            <TextBlock Text="{Binding ErrorMessage}" IsVisible="{Binding HasError}"
+                       Foreground="{DynamicResource ErrorTextBrush}" TextWrapping="Wrap"/>
+            <Button Command="{Binding CreateCommand}" HorizontalAlignment="Left">
+                <StackPanel Orientation="Horizontal" Spacing="6">
+                    <icons:PackIconLucide Kind="Plus" Width="14" Height="14"/>
+                    <TextBlock Text="{x:Static lang:Resources.TagPicker_Create}"/>
+                </StackPanel>
+            </Button>
+        </StackPanel>
+
+        <Button Grid.Row="3" Content="{x:Static lang:Resources.Common_Cancel}" Command="{Binding CancelCommand}"
+                HorizontalAlignment="Right" MinWidth="90" IsCancel="True"/>
+    </Grid>
+</dialogs:DialogWindow>

YKanBan/Views/Dialogs/TagPickerDialog.axaml.cs

@@ -0,0 +1,9 @@
+namespace YKanBan.Views.Dialogs;
+
+public partial class TagPickerDialog : DialogWindow
+{
+    public TagPickerDialog()
+    {
+        InitializeComponent();
+    }
+}

YKanBan/Views/WorkspaceView.axaml

@@ -48,9 +48,11 @@
             </Grid>
         </Border>
 
-        <!-- Tags and board tabs; their contents arrive with the corresponding features. -->
+        <!-- Board and tags tabs; the tags tab is filled in by the tag feature. -->
         <TabControl Grid.Row="1" Margin="8">
-            <TabItem Header="{x:Static lang:Resources.Tab_Board}"/>
+            <TabItem Header="{x:Static lang:Resources.Tab_Board}">
+                <ContentControl Content="{Binding Board}" Margin="0,8,0,0"/>
+            </TabItem>
             <TabItem Header="{x:Static lang:Resources.Tab_Tags}"/>
         </TabControl>
 

docs/PLAN.md

@@ -176,7 +176,7 @@ card_tags: (card_id, tag_id) 复合主键，双向 ON DELETE CASCADE
 **看板选项卡**：
 
 - 搜索栏（点击搜索按钮才执行，无实时搜索，带 × 一键清空）；右侧依次：新增列按钮、列宽调整按钮、排序 combobox
-- 列宽：模态选择预设窄/标准/宽或手动像素值（范围 160–720，默认"标准"= 320），全局生效，持久化到 app.json
+- 列宽：模态选择预设窄/标准/宽或手动像素值（窄 = 240 / 标准 = 320 / 宽 = 480；手动范围 160–720，默认"标准"），全局生效，持久化到 app.json
 - 看板区域（与 GitHub Projects 基本一致）：列左→右横排，卡片上→下展开；**无拖拽**
 - 列内卡片列表必须使用 UI 虚拟化（如 `VirtualizingStackPanel`），满足 10000 张卡片量级的流畅显示
 - 列右上角：＋新建卡片、⋯菜单（编辑、删除）；"编辑" = 标题 + description 编辑模态
@@ -372,8 +372,8 @@ phrase  := '"' 任意文本 '"'   （内部支持转义：\" → 字面引号，
 | M2 仓库层 | 已完成 | CRUD/移动/级联/标签指派、单连接生命周期 | 单测 |
 | M3 搜索与导出 | 已完成 | Pidgin 解析器（严格校验，非法输入返回英文错误消息）、查询→SQL 编译、Markdown 导出（直接读 ResX，en / zh-Hans 输出均有测试） | 单测（成功与错误全分支） |
 | M4 GUI 框架 | 已完成 | 主题/图标接入、主窗口布局（顶部区 + TabControl + 全窗口错误页体系）、设置/关于模态；全 UI 文本资源化（硬规则生效）、语言设置项、RTL/FlowDirection 检测；亮/暗/跟随系统主题基础设施与设置项、ThemeDictionaries 颜色硬规则 | 人工 |
-| **M4R v1.9 回补** | 待办 | 见下方清单 | 本地 Windows build + test 通过；新增页面人工 |
+| **M4R v1.9 回补** | 已完成 | 见下方清单 | 本地 Windows build + test 通过；新增页面人工 |
-| M5 看板界面 | 待办 | 看板 tab 全部交互与模态（含约束冲突模态内提示、卡片列表虚拟化、嵌套模态新建标签立即写库、写库后按最近成功表达式刷新） | 人工 |
+| M5 看板界面 | 已完成（待人工验收） | 看板 tab 全部交互与模态（含约束冲突模态内提示、卡片列表虚拟化、嵌套模态新建标签立即写库、写库后按最近成功表达式刷新） | 人工 |
 | M6 标签与搜索集成 | 待办 | 标签 tab、搜索执行与空列保留、导出接线（含文件名/时间戳 i18n 模式） | 人工 |
 | M7 发布与文档 | 待办 | 单一 workflow（4 RID 矩阵 build→test→artifact，osx-x64 用 Intel 运行器）文件就绪（结构自查）、根目录 USAGE.md、README（引用 USAGE.md）、人工验收清单（含语言切换重启生效、导出语言跟随界面、主题三态切换、app.json 多实例脏写四项） | workflow 结构自查通过；实际触发与产物验证视网络条件推迟 |