fix: bring existing Canvas window to front on tray menu click

Previously, clicking Canvas in the tray menu when the window was already
open did nothing because Activate() was only called when creating a new
window. Move Activate() outside the creation guard so it always runs.

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

src/OpenClaw.Tray.WinUI/Services/NodeService.cs

@@ -1018,8 +1018,8 @@ public sealed class NodeService : IDisposable
         {
             _canvasWindow = new CanvasWindow();
             _canvasWindow.SetTrustedGatewayOrigin(GatewayUrl, _token);
-            _canvasWindow.Activate();
         }
+        _canvasWindow?.Activate();
     }
 
     // Mutable context shared with GatewayActionTransport. SessionKey is updated