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.App/App.axaml.cs

@@ -3,6 +3,7 @@ using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.ApplicationLifetimes;
 using Avalonia.Markup.Xaml;
+using ClaudeDo.Ui;
 using ClaudeDo.Ui.Services;
 using ClaudeDo.Ui.ViewModels;
 using ClaudeDo.Ui.Views;
@@ -22,6 +23,8 @@ public partial class App : Application
     public override void Initialize()
     {
         AvaloniaXamlLoader.Load(this);
+        if (_services?.GetService<AppSettings>() is { } settings)
+            AccentPresetService.Apply(AccentPresets.Find(settings.AccentPreset));
     }
 
     public override void OnFrameworkInitializationCompleted()