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.App/Program.cs

@@ -95,6 +95,7 @@ sealed class Program
 
         // ViewModels
         sc.AddTransient<WorktreeModalViewModel>();
+        sc.AddTransient<Func<WorktreeModalViewModel>>(sp => () => sp.GetRequiredService<WorktreeModalViewModel>());
         sc.AddTransient<WorktreesOverviewModalViewModel>();
         sc.AddTransient<Func<WorktreesOverviewModalViewModel>>(sp => () => sp.GetRequiredService<WorktreesOverviewModalViewModel>());
         sc.AddSingleton<IPrimeScheduleApi, WorkerPrimeScheduleApi>();