fix(ui): fix live output visibility and editor dialog graying out

- Remove wrapping ScrollViewer from live output TextBox — Avalonia
  TextBox with AcceptsReturn handles its own scrolling; nested
  ScrollViewer caused layout collapse
- Auto-scroll via CaretIndex instead of removed ScrollViewer
- Add OnWindowClosed to both editor ViewModels, ensuring the
  TaskCompletionSource always resolves when the dialog closes
  (including via X button), preventing RelayCommand from staying
  permanently disabled

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/ViewModels/TaskListViewModel.cs

@@ -151,6 +151,7 @@ public partial class TaskListViewModel : ViewModelBase
 
         var window = new TaskEditorView { DataContext = editor };
         editor.RequestClose += () => window.Close();
+        window.Closed += (_, _) => editor.OnWindowClosed();
         _ = ShowDialogAsync(window);
 
         var saved = await editor.ShowAndWaitAsync();
@@ -197,6 +198,7 @@ public partial class TaskListViewModel : ViewModelBase
 
         var window = new TaskEditorView { DataContext = editor };
         editor.RequestClose += () => window.Close();
+        window.Closed += (_, _) => editor.OnWindowClosed();
         _ = ShowDialogAsync(window);
 
         var saved = await editor.ShowAndWaitAsync();