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.

backend/app.go

@@ -504,6 +504,12 @@ func (a *App) DeleteServerCacheDir(serverID uuid.UUID) error {
 	return os.RemoveAll(path)
 }
 
+// BackgroundContext returns the application's background context
+// which is canceled when the application shuts down.
+func (a *App) BackgroundContext() context.Context {
+	return a.bgrndCtx
+}
+
 func (a *App) Shutdown() {
 	if a.logFile != nil {
 		a.logFile.Close()