feat(flutter): add auth module with router and screens

- Add app_router.dart with auth routes (login, register)
- Update main.dart with MaterialApp.router
- Add 5 auth screens:
  - LoginEmailScreen with email validation
  - LoginPasswordScreen with password visibility toggle
  - LoginCodeScreen with verification code input
  - RegisterScreen with step indicator
  - RegisterStep2Screen with password/code/invite inputs

apps/lib/core/router/app_router.dart

@@ -0,0 +1,30 @@
+import 'package:go_router/go_router.dart';
+
+import '../../features/auth/ui/screens/login_email_screen.dart';
+import '../../features/auth/ui/screens/login_password_screen.dart';
+import '../../features/auth/ui/screens/login_code_screen.dart';
+import '../../features/auth/ui/screens/register_screen.dart';
+import '../../features/auth/ui/screens/register_step2_screen.dart';
+
+final appRouter = GoRouter(
+  initialLocation: '/',
+  routes: [
+    GoRoute(path: '/', builder: (context, state) => const LoginEmailScreen()),
+    GoRoute(
+      path: '/login/password',
+      builder: (context, state) => const LoginPasswordScreen(),
+    ),
+    GoRoute(
+      path: '/login/code',
+      builder: (context, state) => const LoginCodeScreen(),
+    ),
+    GoRoute(
+      path: '/register',
+      builder: (context, state) => const RegisterScreen(),
+    ),
+    GoRoute(
+      path: '/register/step2',
+      builder: (context, state) => const RegisterStep2Screen(),
+    ),
+  ],
+);