feat: add auth screens and state management

lib/app.dart

@@ -0,0 +1,28 @@
+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),
+    );
+  }
+}