fix: null _gatewayClient after dispose in OnSettingsSaved

When OnSettingsSaved switches to node mode, _gatewayClient was
disposed but not set to null. This caused guards like
  if (_gatewayClient == null) return;
in ShowQuickSend, RunHealthCheckAsync, and ToggleChannel to pass
instead of short-circuiting, potentially calling into a disposed
object (ObjectDisposedException) or producing unexpected behaviour.

The _nodeService field already followed the correct pattern
(null first, then dispose). Apply the same pattern to _gatewayClient.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/OpenClaw.Tray.WinUI/App.xaml.cs

@@ -1606,6 +1606,7 @@ public partial class App : Application
         // Reconnect with new settings — mirror the startup if/else pattern
         // to avoid dual connections that cause gateway conflicts.
         _gatewayClient?.Dispose();
+        _gatewayClient = null;
         var oldNodeService = _nodeService;
         _nodeService = null;
         try { oldNodeService?.Dispose(); } catch (Exception ex) { Logger.Warn($"Node dispose error: {ex.Message}"); }