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/Services/AccentPresetService.cs

@@ -0,0 +1,24 @@
+using Avalonia;
+using Avalonia.Media;
+
+namespace ClaudeDo.Ui.Services;
+
+public static class AccentPresetService
+{
+    public static void Apply(AccentPreset preset)
+    {
+        if (Application.Current is not { } app) return;
+
+        SetBrushColor(app, "AccentBrush",     preset.Accent);
+        SetBrushColor(app, "AccentDimBrush",  preset.Dim);
+        SetBrushColor(app, "AccentSoftBrush", preset.Soft);
+        SetBrushColor(app, "AccentGlowBrush", preset.Glow);
+        SetBrushColor(app, "MossBrush",       preset.Accent);
+    }
+
+    private static void SetBrushColor(Application app, string key, string hex)
+    {
+        if (app.TryGetResource(key, null, out var value) && value is SolidColorBrush brush)
+            brush.Color = Color.Parse(hex);
+    }
+}