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/MainWindow.axaml.cs

@@ -3,6 +3,7 @@ using Avalonia.Controls;
 using Avalonia.Input;
 using ClaudeDo.Ui.ViewModels;
 using ClaudeDo.Ui.ViewModels.Islands;
+using ClaudeDo.Ui.ViewModels.Modals;
 using ClaudeDo.Ui.Views.Modals;
 using ClaudeDo.Ui.Views.Planning;
 
@@ -45,7 +46,13 @@ public partial class MainWindow : Window
                         if (item is not null && s.Lists is not null) s.Lists.SelectedList = item;
                     }
                 };
-                // TODO: ShowDiffAction and ConfirmAction not wired in v1
+                modal.ShowDiffAction = diffVm =>
+                {
+                    var diffDlg = new WorktreeModalView { DataContext = diffVm };
+                    diffVm.CloseAction = () => diffDlg.Close();
+                    _ = diffVm.LoadAsync();
+                    _ = diffDlg.ShowDialog(this);
+                };
                 await dlg.ShowDialog(this);
             };
         }