feat: add auth screens and state management

lib/main.dart

@@ -0,0 +1,9 @@
+import 'package:firebase_core/firebase_core.dart';
+import 'package:flutter/material.dart';
+import 'package:phone_login/app.dart';
+
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  await Firebase.initializeApp();
+  runApp(const App());
+}