fix(ui): cancel retry loop before disposing worker connection

Add _startCts?.Cancel() at the top of DisposeAsync to prevent
ConnectWithRetryAsync from calling _hub.StartAsync on an already-disposed
HubConnection when the caller disposes without first calling StopAsync.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -182,6 +182,7 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable
 
     public async ValueTask DisposeAsync()
     {
+        _startCts?.Cancel();
         await _hub.DisposeAsync();
     }