fix(ui): harden worker auto-reconnect lifecycle

- Fix _startCts lifecycle leak: old CTS is cancelled and disposed before replacement
- Guard StartAsync re-entrancy: early-return if retry loop is still running (lock + IsCompleted check)
- Await _retryLoopTask in both StopAsync and DisposeAsync before stopping/disposing the hub

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

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -26,6 +26,8 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable
 {
     private readonly HubConnection _hub;
     private CancellationTokenSource? _startCts;
+    private Task _retryLoopTask = Task.CompletedTask;
+    private readonly object _startLock = new();
 
     [ObservableProperty]
     private bool _isConnected;
@@ -109,8 +111,18 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable
 
     public Task StartAsync()
     {
+        lock (_startLock)
+        {
+            if (!_retryLoopTask.IsCompleted)
+                return Task.CompletedTask;
+
+            var old = _startCts;
             _startCts = new CancellationTokenSource();
-        _ = ConnectWithRetryAsync(_startCts.Token);
+            old?.Cancel();
+            old?.Dispose();
+
+            _retryLoopTask = ConnectWithRetryAsync(_startCts.Token);
+        }
         return Task.CompletedTask;
     }
 
@@ -144,6 +156,7 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable
     public async Task StopAsync()
     {
         _startCts?.Cancel();
+        try { await _retryLoopTask; } catch (OperationCanceledException) { } catch { /* swallow */ }
         try { await _hub.StopAsync(); } catch { /* swallow */ }
     }
 
@@ -183,6 +196,7 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable
     public async ValueTask DisposeAsync()
     {
         _startCts?.Cancel();
+        try { await _retryLoopTask; } catch (OperationCanceledException) { } catch { /* swallow */ }
         await _hub.DisposeAsync();
     }