fix(ui): make worktree state chips readable with on-theme tints

The state badge in the worktrees overview used bright off-palette Material colors
with hardcoded near-black text (via WorktreeStateColorConverter), which was hard
to read. Switch to the existing chip pattern (subtle tint background + matching
border + colored text): active=blue, merged=green, kept=amber, discarded=gray.
Drop the now-unused WorktreeStateColorConverter.

src/ClaudeDo.Ui/Converters/WorktreeStateColorConverter.cs

@@ -1,30 +0,0 @@
-using System.Globalization;
-using Avalonia.Data.Converters;
-using Avalonia.Media;
-using ClaudeDo.Data.Models;
-
-namespace ClaudeDo.Ui.Converters;
-
-public sealed class WorktreeStateColorConverter : IValueConverter
-{
-    private static readonly ISolidColorBrush Active = new SolidColorBrush(Color.Parse("#42A5F5"));
-    private static readonly ISolidColorBrush Merged = new SolidColorBrush(Color.Parse("#66BB6A"));
-    private static readonly ISolidColorBrush Discarded = new SolidColorBrush(Color.Parse("#9E9E9E"));
-    private static readonly ISolidColorBrush Kept = new SolidColorBrush(Color.Parse("#FFA726"));
-    private static readonly ISolidColorBrush Default = new SolidColorBrush(Colors.Gray);
-
-    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture) =>
-        value is WorktreeState state
-            ? state switch
-            {
-                WorktreeState.Active    => Active,
-                WorktreeState.Merged    => Merged,
-                WorktreeState.Discarded => Discarded,
-                WorktreeState.Kept      => Kept,
-                _                       => Default,
-            }
-            : Default;
-
-    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
-        => throw new NotSupportedException();
-}