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/AppSettings.cs

@@ -8,6 +8,7 @@ public sealed class AppSettings
     public string DbPath { get; set; } = "~/.todo-app/todo.db";
     public string SignalRUrl { get; set; } = "http://127.0.0.1:47821/hub";
     public string Language { get; set; } = "";
+    public string AccentPreset { get; set; } = "";
 
     private static readonly string ConfigPath = Paths.Expand("~/.todo-app/ui.config.json");