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/Islands/ListsIslandView.axaml.cs

@@ -38,7 +38,15 @@ public partial class ListsIslandView : UserControl
                             if (item is not null) v.SelectedList = item;
                         }
                     };
-                    // TODO: ShowDiffAction and ConfirmAction not wired in v1
+                    modal.ShowDiffAction = diffVm =>
+                    {
+                        var top2 = TopLevel.GetTopLevel(this) as Window;
+                        if (top2 is null) return;
+                        var dlg = new WorktreeModalView { DataContext = diffVm };
+                        diffVm.CloseAction = () => dlg.Close();
+                        _ = diffVm.LoadAsync();
+                        _ = dlg.ShowDialog(top2);
+                    };
                     var top = TopLevel.GetTopLevel(this) as Window;
                     if (top is null) window.Show();
                     else await window.ShowDialog(top);