Fix WidgetPool goroutine and ticker leak

- Add context cancellation to WidgetPool cleanup goroutine
- Properly stop ticker when context is canceled
- Add BackgroundContext() getter method to App for context access
- Pass application context to NewWidgetPool for lifecycle management

This fixes a resource leak where the cleanup goroutine would run
indefinitely without any way to stop it, preventing proper cleanup
on application shutdown.

ui/browsing/router.go

@@ -26,7 +26,7 @@ func NewRouter(app *backend.App, controller *controller.Controller, nav Navigati
 		App:        app,
 		Controller: controller,
 		Nav:        nav,
-		widgetPool: util.NewWidgetPool(),
+		widgetPool: util.NewWidgetPool(app.BackgroundContext()),
 	}
 	return r
 }