feat: add auth screens and state management

lib/app.dart

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+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+import 'package:phone_login/auth/auth_state.dart';
+import 'package:phone_login/auth/phone_input_screen.dart';
+import 'package:phone_login/auth/sms_verification_screen.dart';
+import 'package:provider/provider.dart';
+
+final _router = GoRouter(
+  routes: [
+    GoRoute(path: '/', builder: (context, state) => const PhoneInputScreen()),
+    GoRoute(
+      path: '/sms_verify',
+      builder: (context, state) => const SmsVerificationScreen(),
+    ),
+  ],
+);
+
+class App extends StatelessWidget {
+  const App({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return ChangeNotifierProvider(
+      create: (_) => AuthState(),
+      child: MaterialApp.router(routerConfig: _router),
+    );
+  }
+}

lib/auth/auth_state.dart

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+import 'package:flutter/material.dart';
+import 'package:firebase_auth/firebase_auth.dart';
+
+class AuthState extends ChangeNotifier {
+  User? _user;
+  User? get user => _user;
+
+  final FirebaseAuth _auth = FirebaseAuth.instance;
+
+  AuthState() {
+    _auth.authStateChanges().listen((user) {
+      _user = user;
+      notifyListeners();
+    });
+  }
+
+  bool get isLoggedIn => _user != null;
+}

lib/auth/phone_input_screen.dart

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+import 'package:flutter/material.dart';
+import 'package:intl_phone_field/intl_phone_field.dart';
+
+class PhoneInputScreen extends StatelessWidget {
+  const PhoneInputScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('Enter Phone Number')),
+      body: Padding(
+        padding: const EdgeInsets.all(16.0),
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.stretch,
+          children: [
+            IntlPhoneField(
+              decoration: const InputDecoration(
+                labelText: 'Phone Number',
+                border: OutlineInputBorder(borderSide: BorderSide()),
+              ),
+              initialCountryCode: 'US',
+              onChanged: (phone) {
+                // TODO: Handle phone number changes
+              },
+            ),
+            const SizedBox(height: 20),
+            ElevatedButton(onPressed: () {}, child: const Text('Send OTP')),
+          ],
+        ),
+      ),
+    );
+  }
+}

lib/auth/sms_verification_screen.dart

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+import 'package:flutter/material.dart';
+import 'package:pinput/pinput.dart';
+
+class SmsVerificationScreen extends StatelessWidget {
+  const SmsVerificationScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('SMS Verification')),
+      body: Padding(
+        padding: const EdgeInsets.all(16.0),
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.stretch,
+          children: [
+            const Text(
+              'Enter the 6-digit code sent to you',
+              textAlign: TextAlign.center,
+              style: TextStyle(fontSize: 16),
+            ),
+            const SizedBox(height: 20),
+            Pinput(
+              length: 6,
+              onCompleted: (pin) {
+                // TODO: Handle OTP completion
+              },
+            ),
+            const SizedBox(height: 20),
+            ElevatedButton(onPressed: () {}, child: const Text('Verify')),
+          ],
+        ),
+      ),
+    );
+  }
+}

lib/main.dart

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+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());
+}