feat(ui): polish worktrees overview modal

- Restyle to match ListSettingsModalView: custom title bar, DeepBrush
  toolbar, LineBrush footer, SurfaceBrush outer border, no system chrome
- Add column header row (TASK / BRANCH / STATE / DIFF / AGE) with
  TextFaintBrush + MonoFont + LetterSpacing, separator line below
- Replace wt-row style with task-row-equivalent: transparent bg,
  CornerRadius 8, 1px border, :pointerover + .selected transitions
- Add IsSelected to WorktreeOverviewRowViewModel; SelectRow() helper
  on modal VM clears previous selection before setting new one
- Wire OnRowTapped in code-behind for click-to-select
- Wire ShowDiff: VM takes Func<WorktreeModalViewModel> factory, builds
  diffVm and delegates window creation to both call sites (MainWindow
  and ListsIslandView); register Func<WorktreeModalViewModel> in DI

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Modals/WorktreesOverviewModalView.axaml.cs

@@ -1,8 +1,25 @@
 using Avalonia.Controls;
+using Avalonia.Input;
+using ClaudeDo.Ui.ViewModels.Modals;
 
 namespace ClaudeDo.Ui.Views.Modals;
 
 public partial class WorktreesOverviewModalView : Window
 {
     public WorktreesOverviewModalView() => InitializeComponent();
+
+    private void OnRowTapped(object? sender, TappedEventArgs e)
+    {
+        if (sender is Border { DataContext: WorktreeOverviewRowViewModel row } &&
+            DataContext is WorktreesOverviewModalViewModel vm)
+        {
+            vm.SelectRow(row);
+        }
+    }
+
+    private void OnTitleBarPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
+            BeginMoveDrag(e);
+    }
 }