fix(ui): prevent async void races and leak-on-exit

- task detail vm: LoadAsync now uses a per-call CancellationTokenSource
  so rapid TaskUpdated events can't race on _taskId / Subtasks / Tags;
  old subtask PropertyChanged handlers are torn down before Clear
- task detail vm: async void event handlers (OnTaskUpdated,
  OnWorktreeUpdated, OnSubtaskPropertyChanged) wrap work in try/catch
  so thrown exceptions can't crash the Avalonia sync context
- task detail vm: Clear cancels/disposes the load CTS so a late-arriving
  LoadAsync can't resurrect detail state after deselect
- app: DisposeAsync the ServiceProvider in a finally after the classic
  desktop lifetime ends, so WorkerClient.DisposeAsync runs and the
  SignalR connection closes cleanly instead of being abandoned

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

src/ClaudeDo.App/Program.cs

@@ -22,8 +22,19 @@ sealed class Program
         var factory = services.GetRequiredService<SqliteConnectionFactory>();
         SchemaInitializer.Apply(factory);
 
-        BuildAvaloniaApp()
-            .StartWithClassicDesktopLifetime(args);
+        try
+        {
+            BuildAvaloniaApp()
+                .StartWithClassicDesktopLifetime(args);
+        }
+        finally
+        {
+            // Dispose the container so WorkerClient.DisposeAsync runs —
+            // cancels the retry loop and closes the SignalR connection cleanly
+            // instead of abandoning it.
+            try { services.DisposeAsync().AsTask().GetAwaiter().GetResult(); }
+            catch { /* best effort on shutdown */ }
+        }
     }
 
     public static AppBuilder BuildAvaloniaApp()