fix(ui): address code review findings

- Fix event handler leak in TaskDetailView.OnDataContextChanged
  (unsubscribe from previous ViewModel before subscribing to new one)
- Move FormatFile call off UI thread with Task.Run to prevent freeze
  on large log files

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

src/ClaudeDo.Ui/ViewModels/TaskDetailViewModel.cs

@@ -90,7 +90,7 @@ public partial class TaskDetailViewModel : ViewModelBase
                 && File.Exists(task.LogPath))
             {
                 _formatter = new StreamLineFormatter();
-                LiveText = _formatter.FormatFile(task.LogPath);
+                LiveText = await Task.Run(() => _formatter.FormatFile(task.LogPath));
             }
             StatusText = task.Status.ToString().ToLowerInvariant();
             StatusChoice = task.Status.ToString();