fix(settings): apply a theme as soon as it is selected and restore it on cancel

PLAN 5.6 asks for the theme to take effect immediately, but it was applied
only on OK. Selecting a theme now previews it; OK saves it, and cancel, Escape
or closing the window restores the theme the dialog opened with.

YKanBan.Tests/ViewModels/SettingsDialogViewModelTests.cs

@@ -0,0 +1,63 @@
+using YKanBan.Storage.AppData;
+using YKanBan.ViewModels.Dialogs;
+
+namespace YKanBan.Tests.ViewModels;
+
+/// <summary>
+/// Tests for the settings dialog: the theme is previewed as soon as it is
+/// selected, saved on accept and restored on cancel.
+/// </summary>
+[TestClass]
+public class SettingsDialogViewModelTests
+{
+    [TestMethod]
+    public void OpeningTheDialogAppliesNothing()
+    {
+        var applied = new List<ThemeOption>();
+
+        _ = new SettingsDialogViewModel(new AppConfig(), AppConfig.DefaultLanguage, applied.Add);
+
+        Assert.AreEqual(0, applied.Count);
+    }
+
+    [TestMethod]
+    public void SelectingAThemeAppliesItImmediatelyWithoutSavingIt()
+    {
+        var config = new AppConfig();
+        var applied = new List<ThemeOption>();
+        var dialog = new SettingsDialogViewModel(config, AppConfig.DefaultLanguage, applied.Add);
+
+        dialog.SelectedTheme = dialog.Themes.Single(option => option.Value.Equals(ThemeOption.Dark));
+
+        CollectionAssert.AreEqual(new[] { ThemeOption.Dark }, applied);
+        Assert.AreEqual(ThemeOption.FollowSystem, config.Theme);
+    }
+
+    [TestMethod]
+    public void AcceptSavesThePreviewedTheme()
+    {
+        var config = new AppConfig();
+        var applied = new List<ThemeOption>();
+        var dialog = new SettingsDialogViewModel(config, AppConfig.DefaultLanguage, applied.Add);
+        dialog.SelectedTheme = dialog.Themes.Single(option => option.Value.Equals(ThemeOption.Light));
+
+        dialog.Accept();
+
+        Assert.AreEqual(ThemeOption.Light, config.Theme);
+        CollectionAssert.AreEqual(new[] { ThemeOption.Light }, applied);
+    }
+
+    [TestMethod]
+    public void CancelRestoresTheThemeTheDialogOpenedWith()
+    {
+        var config = new AppConfig { Theme = ThemeOption.Light };
+        var applied = new List<ThemeOption>();
+        var dialog = new SettingsDialogViewModel(config, AppConfig.DefaultLanguage, applied.Add);
+        dialog.SelectedTheme = dialog.Themes.Single(option => option.Value.Equals(ThemeOption.Dark));
+
+        dialog.Cancel();
+
+        Assert.AreEqual(ThemeOption.Light, config.Theme);
+        CollectionAssert.AreEqual(new[] { ThemeOption.Dark, ThemeOption.Light }, applied);
+    }
+}

YKanBan/ViewModels/Dialogs/SettingsDialogViewModel.cs

@@ -5,23 +5,27 @@ namespace YKanBan.ViewModels.Dialogs;
 
 /// <summary>
 /// The settings dialog: language (restart to apply), theme and the four
-/// confirmation-dialog toggles. Nothing is applied until the user accepts;
-/// cancelling or closing the window discards the changes.
+/// confirmation-dialog toggles. A theme selection is previewed immediately;
+/// everything else waits until the user accepts. Cancelling or closing the
+/// window discards the changes and restores the theme the dialog opened with.
 /// </summary>
 public sealed partial class SettingsDialogViewModel : ViewModelBase
 {
     private readonly AppConfig _config;
     private readonly string _startupLanguage;
+    private readonly Action<ThemeOption> _applyTheme;
 
     /// <summary>
     /// Initializes the dialog from the live configuration.
     /// </summary>
     /// <param name="config">The live application configuration.</param>
     /// <param name="startupLanguage">The language applied at startup, for the restart notice.</param>
-    public SettingsDialogViewModel(AppConfig config, string startupLanguage)
+    /// <param name="applyTheme">Applies a theme to the running app; defaults to <see cref="App.ApplyTheme"/>.</param>
+    public SettingsDialogViewModel(AppConfig config, string startupLanguage, Action<ThemeOption>? applyTheme = null)
     {
         _config = config;
         _startupLanguage = startupLanguage;
+        _applyTheme = applyTheme ?? App.ApplyTheme;
 
         Languages =
         [
@@ -63,7 +67,8 @@ public sealed partial class SettingsDialogViewModel : ViewModelBase
     private OptionItem? selectedLanguage;
 
     /// <summary>
-    /// Gets or sets the selected theme; applied when the dialog is accepted.
+    /// Gets or sets the selected theme; applied to the running app as soon as it
+    /// changes, and saved to the configuration when the dialog is accepted.
     /// </summary>
     [ObservableProperty]
     private OptionItem? selectedTheme;
@@ -110,7 +115,6 @@ public sealed partial class SettingsDialogViewModel : ViewModelBase
         if (SelectedTheme is not null)
         {
             _config.Theme = (ThemeOption)SelectedTheme.Value;
-            App.ApplyTheme(_config.Theme);
         }
 
         _config.Confirmations.DeleteCard = ConfirmDeleteCard;
@@ -119,6 +123,23 @@ public sealed partial class SettingsDialogViewModel : ViewModelBase
         _config.Confirmations.DiscardEdit = ConfirmDiscardEdit;
     }
 
+    /// <summary>
+    /// Discards the working values and restores the theme the dialog opened with.
+    /// </summary>
+    public void Cancel() => _applyTheme(_config.Theme);
+
+    /// <summary>
+    /// Previews a theme selection on the running app.
+    /// </summary>
+    /// <param name="value">The new selection.</param>
+    partial void OnSelectedThemeChanged(OptionItem? value)
+    {
+        if (value is not null)
+        {
+            _applyTheme((ThemeOption)value.Value);
+        }
+    }
+
     /// <summary>
     /// Refreshes the restart notice after a language selection change.
     /// </summary>

YKanBan/Views/Dialogs/SettingsDialog.axaml.cs

@@ -6,6 +6,8 @@ namespace YKanBan.Views.Dialogs;
 
 public partial class SettingsDialog : Window
 {
+    private bool _accepted;
+
     public SettingsDialog()
     {
         InitializeComponent();
@@ -15,9 +17,24 @@ public partial class SettingsDialog : Window
     private void OnOkClick(object? sender, RoutedEventArgs e)
     {
         (DataContext as SettingsDialogViewModel)?.Accept();
+        _accepted = true;
         Close(true);
     }
 
     /// <summary>Closes without applying anything.</summary>
     private void OnCancelClick(object? sender, RoutedEventArgs e) => Close(false);
+
+    /// <summary>
+    /// Restores the previewed theme on every close that did not accept,
+    /// including the title-bar close button and Escape.
+    /// </summary>
+    /// <param name="e">The event arguments.</param>
+    protected override void OnClosed(EventArgs e)
+    {
+        if (!_accepted)
+        {
+            (DataContext as SettingsDialogViewModel)?.Cancel();
+        }
+        base.OnClosed(e);
+    }
 }

docs/ACCEPTANCE.md

@@ -30,7 +30,8 @@
 
 ### A3 主题三态切换
 
-- [ ] 设置 → 主题 亮色 / 暗色 / 跟随系统 依次切换：**即时生效**，无重启提示
+- [ ] 设置 → 主题 亮色 / 暗色 / 跟随系统 依次切换：下拉框一选中就**即时生效**（无需点确认），无重启提示
+- [ ] 切换主题后点取消、按 Esc 或点标题栏关闭：恢复为打开设置时的主题；重启后仍是原主题
 - [ ] 每种主题下检查：主窗口、看板列与卡片、标签 tab、全部模态、全部错误页颜色完整切换，无亮底暗字/暗底亮字残留
 - [ ] 标签徽章在两种主题下均显示原色（§5.9 例外），文字颜色保持可读
 - [ ] 跟随系统：程序运行中切换 Windows 深浅色 → 程序跟随变化

docs/PLAN.md

@@ -214,7 +214,7 @@ card_tags: (card_id, tag_id) 复合主键，双向 ON DELETE CASCADE
 入口 = 顶部显示区域右侧"设置"按钮，承载各类全局设置：
 
 - 语言选择（英文 / 简体中文），切换后显示"重启后生效"提示（提示文本本身为资源键）
-- 主题选择（亮色 / 暗色 / 跟随系统），切换后**即时生效**，无重启提示（与语言项的行为差异）
+- 主题选择（亮色 / 暗色 / 跟随系统），下拉框选中即**即时生效**（预览），无重启提示（与语言项的行为差异）；点确认才写入配置，取消或关闭设置模态则恢复打开时的主题
 - 四类确认开关（见 §5.5）
 
 ### 5.7 关于模态