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/AccentPresets.cs

@@ -0,0 +1,43 @@
+using Avalonia.Media;
+using ClaudeDo.Ui.Localization;
+using ClaudeDo.Ui.ViewModels;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace ClaudeDo.Ui;
+
+public sealed record AccentPreset(string Name, string Accent, string Dim, string Soft, string Glow);
+
+public static class AccentPresets
+{
+    // Hue 88 — moss/sage green (original)
+    public static readonly AccentPreset Moss = new("moss", "#FF7C9166", "#FF64785A", "#FF3E4B39", "#387C9166");
+    // Hue ~40 — warm earthy brown/terra
+    public static readonly AccentPreset Peat = new("peat", "#FF9A7B5C", "#FF7F6449", "#FF4D3C2C", "#389A7B5C");
+    // Hue ~180 — cool teal/sea-green
+    public static readonly AccentPreset Sea  = new("sea",  "#FF5B8F8C", "#FF4A7573", "#FF263D3C", "#385B8F8C");
+
+    public static readonly IReadOnlyList<AccentPreset> All = [Moss, Peat, Sea];
+    public static AccentPreset Default => Moss;
+
+    public static AccentPreset Find(string? name) =>
+        All.FirstOrDefault(p => p.Name == name) ?? Default;
+}
+
+public sealed partial class AccentPresetSwatchViewModel : ViewModelBase
+{
+    public AccentPreset Preset { get; }
+
+    [ObservableProperty] private bool _isSelected;
+
+    public Color DisplayColor { get; }
+
+    public string DisplayName =>
+        Loc.T($"settings.general.accentPreset{char.ToUpperInvariant(Preset.Name[0])}{Preset.Name.Substring(1)}");
+
+    public AccentPresetSwatchViewModel(AccentPreset preset, bool selected)
+    {
+        Preset = preset;
+        _isSelected = selected;
+        DisplayColor = Color.Parse(preset.Accent);
+    }
+}