feat(m1): storage layer

- Shared SQLite plumbing: PRAGMAs (WAL/NORMAL/foreign_keys) and
  user_version migrations with typed SchemaVersionException
- Workspace folder: .ykanban initialization, empty schema with
  DDL-level constraints, preset column seeding (ResX-driven),
  exclusive lock file with holder diagnostics
- AppData folder: app.db workspace registry (injectable clock),
  app.json config store with kebab-case JSON names via explicit
  [JsonProperty]/[EnumMember], nested sections, corrupt-file backup
- ResX infrastructure under Assets/Locales (en + zh-Hans) with
  MSBuild strongly-typed public Resources class
- 54 unit tests: DDL constraints, migrations, PRAGMAs, lock
  exclusivity, config degradation, culture-aware presets

YKanBan.Tests/Storage/AppData/AppConfigStoreTests.cs

@@ -0,0 +1,169 @@
+using YKanBan.Storage.AppData;
+using YKanBan.Tests.TestUtilities;
+
+namespace YKanBan.Tests.Storage.AppData;
+
+/// <summary>Tests for app.json load/save and its degradation rules.</summary>
+[TestClass]
+public class AppConfigStoreTests
+{
+    [TestMethod]
+    public void MissingFileYieldsDefaults()
+    {
+        using var directory = new TempDirectory();
+        var store = new AppConfigStore(Path.Combine(directory.FullPath, "app.json"));
+
+        AppConfig config = store.Load();
+        Assert.AreEqual(AppConfig.CurrentFormatVersion, config.Version);
+        Assert.AreEqual("en", config.Language);
+        Assert.AreEqual(ThemeOption.FollowSystem, config.Theme);
+        Assert.AreEqual(CardSortOption.Id, config.Sort.Card);
+        Assert.AreEqual(SidebarSortOption.NameAsc, config.Sort.Sidebar);
+        Assert.AreEqual(ColumnWidthPreset.Standard, config.ColumnWidth.Preset);
+        Assert.AreEqual(320, config.ColumnWidth.CustomPixels);
+        Assert.IsTrue(config.Confirmations.DeleteCard);
+        Assert.IsTrue(config.Confirmations.DeleteColumn);
+        Assert.IsTrue(config.Confirmations.DeleteTag);
+        Assert.IsTrue(config.Confirmations.DiscardEdit);
+    }
+
+    [TestMethod]
+    public void CorruptJsonIsBackedUpAndDefaultsAreUsed()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        File.WriteAllText(path, "{{{ not json");
+        var store = new AppConfigStore(path);
+
+        AppConfig config = store.Load();
+
+        Assert.AreEqual("en", config.Language);
+        Assert.IsTrue(File.Exists(path + ".bak"));
+        Assert.AreEqual("{{{ not json", File.ReadAllText(path + ".bak"));
+    }
+
+    [TestMethod]
+    public void WrongShapeJsonIsBackedUpAndDefaultsAreUsed()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        File.WriteAllText(path, "[1, 2, 3]");
+        var store = new AppConfigStore(path);
+
+        AppConfig config = store.Load();
+
+        Assert.AreEqual(AppConfig.CurrentFormatVersion, config.Version);
+        Assert.IsTrue(File.Exists(path + ".bak"));
+    }
+
+    [TestMethod]
+    public void SaveAndLoadRoundTripsAllSettings()
+    {
+        using var directory = new TempDirectory();
+        var store = new AppConfigStore(Path.Combine(directory.FullPath, "app.json"));
+
+        store.Save(new AppConfig
+        {
+            Language = "zh-Hans",
+            Theme = ThemeOption.Dark,
+            Sort = new SortSettings
+            {
+                Card = CardSortOption.Title,
+                Sidebar = SidebarSortOption.RecentlyUpdated,
+            },
+            ColumnWidth = new ColumnWidthSettings
+            {
+                Preset = ColumnWidthPreset.Custom,
+                CustomPixels = 500,
+            },
+            Confirmations = new ConfirmationSettings
+            {
+                DeleteCard = false,
+                DeleteColumn = false,
+                DeleteTag = false,
+                DiscardEdit = false,
+            },
+        });
+        AppConfig loaded = store.Load();
+
+        Assert.AreEqual("zh-Hans", loaded.Language);
+        Assert.AreEqual(ThemeOption.Dark, loaded.Theme);
+        Assert.AreEqual(CardSortOption.Title, loaded.Sort.Card);
+        Assert.AreEqual(SidebarSortOption.RecentlyUpdated, loaded.Sort.Sidebar);
+        Assert.AreEqual(ColumnWidthPreset.Custom, loaded.ColumnWidth.Preset);
+        Assert.AreEqual(500, loaded.ColumnWidth.CustomPixels);
+        Assert.IsFalse(loaded.Confirmations.DeleteCard);
+        Assert.IsFalse(loaded.Confirmations.DeleteColumn);
+        Assert.IsFalse(loaded.Confirmations.DeleteTag);
+        Assert.IsFalse(loaded.Confirmations.DiscardEdit);
+    }
+
+    [TestMethod]
+    public void SaveWritesKebabCaseNamesAndVersionField()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        var store = new AppConfigStore(path);
+
+        store.Save(new AppConfig());
+        string json = File.ReadAllText(path);
+
+        // Keys and enum values are serialized in kebab-case; "version" is the migration hook.
+        Assert.IsTrue(json.Contains($"\"version\": {AppConfig.CurrentFormatVersion}"));
+        Assert.IsTrue(json.Contains("\"language\": \"en\""));
+        Assert.IsTrue(json.Contains("\"theme\": \"follow-system\""));
+        Assert.IsTrue(json.Contains("\"sort\":"));
+        Assert.IsTrue(json.Contains("\"card\": \"id\""));
+        Assert.IsTrue(json.Contains("\"sidebar\": \"name-asc\""));
+        Assert.IsTrue(json.Contains("\"column-width\":"));
+        Assert.IsTrue(json.Contains("\"preset\": \"standard\""));
+        Assert.IsTrue(json.Contains("\"custom-pixels\": 320"));
+        Assert.IsTrue(json.Contains("\"confirmations\":"));
+        Assert.IsTrue(json.Contains("\"delete-card\": true"));
+        Assert.IsTrue(json.Contains("\"delete-column\": true"));
+        Assert.IsTrue(json.Contains("\"delete-tag\": true"));
+        Assert.IsTrue(json.Contains("\"discard-edit\": true"));
+    }
+
+    [TestMethod]
+    public void MissingSectionsFallBackToTheirDefaults()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        File.WriteAllText(path, $$"""
+            {
+              "version": {{AppConfig.CurrentFormatVersion}},
+              "language": "zh-Hans"
+            }
+            """);
+        var store = new AppConfigStore(path);
+
+        AppConfig config = store.Load();
+
+        // The present field loads; absent sections each fall back to defaults.
+        Assert.AreEqual("zh-Hans", config.Language);
+        Assert.AreEqual(CardSortOption.Id, config.Sort.Card);
+        Assert.AreEqual(ColumnWidthPreset.Standard, config.ColumnWidth.Preset);
+        Assert.IsTrue(config.Confirmations.DeleteCard);
+    }
+
+    [TestMethod]
+    public void LoadPreservesUnknownFieldsTolerantly()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        // A newer version writes an unknown field plus a known one.
+        File.WriteAllText(path, $$"""
+            {
+              "version": {{AppConfig.CurrentFormatVersion}},
+              "language": "zh-Hans",
+              "some-future-field": 42
+            }
+            """);
+        var store = new AppConfigStore(path);
+
+        AppConfig config = store.Load();
+
+        Assert.AreEqual("zh-Hans", config.Language);
+    }
+}