feat(ui): accent color presets in Settings → General

Adds Moss / Peat / Sea preset swatches to the General settings tab.
Selecting a preset mutates the live SolidColorBrush objects in the
Application resource dictionary so all StaticResource consumers update
instantly; the choice is persisted to ui.config.json and re-applied
at startup. Missing or unknown saved value falls back to Moss.

src/ClaudeDo.Ui/ViewModels/Modals/SettingsModalViewModel.cs

@@ -36,6 +36,11 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
             appSettings.Language = code;
             appSettings.Save();
         });
+        General.InitAccentPresets(appSettings.AccentPreset, preset =>
+        {
+            appSettings.AccentPreset = preset;
+            appSettings.Save();
+        });
         Worktrees = new WorktreesSettingsTabViewModel(worker);
         Files = new FilesSettingsTabViewModel(worker);
         Prime = prime;