refactor(diff): single DiffViewer replaces DiffModal + WorktreeModal + PlanningDiff

src/ClaudeDo.Ui/Views/WindowDialogService.cs

@@ -73,7 +73,7 @@ public sealed class WindowDialogService : IDialogService
         };
         vm.ShowDiffAction = diffVm =>
         {
-            var diffDlg = new WorktreeModalView { DataContext = diffVm };
+            var diffDlg = new DiffViewerView { DataContext = diffVm };
             diffVm.CloseAction = () => diffDlg.Close();
             _ = diffVm.LoadAsync();
             _ = diffDlg.ShowDialog(_owner);