main function
The entry point of the Flutter application.
Initializes Firebase, sets up FCM background message handling, and runs the app.
Returns: A Future<void> that completes when initialization is done.
Implementation
Future<void> main() async {
WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp();
FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);
runApp(const MyApp());
}