initialize method
Initializes Firebase core. Must be called before using FirebaseAuth.
Optionally accepts custom FirebaseOptions for web or custom config.
Implementation
Future<void> initialize({FirebaseOptions? options}) async {
if (_ready) return;
// Ensure Flutter widgets binding is initialized (required for Firebase).
WidgetsFlutterBinding.ensureInitialized();
// Initialize Firebase with optional config.
await Firebase.initializeApp(options: options);
_ready = true;
}