ensureInitialized static method
Initializes local notifications and attaches FCM foreground listeners.
Call once in main after Firebase.initializeApp. This is the single
SDK entry point for foreground push handling — host apps must not call
attachFirebaseListeners separately.
For the FCM background isolate, the SDK uses ensureTrayDisplayReady instead (no permission prompt, no FCM listener attachment).
Implementation
static Future<void> ensureInitialized() async {
await ensureTrayDisplayReady(background: false);
_attachFirebaseListenersOnce();
_initialized = true;
}