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/main.dart

@@ -0,0 +1,21 @@
+import 'package:flutter/material.dart';
+import 'core/theme/app_theme.dart';
+import 'core/router/app_router.dart';
+
+void main() {
+  runApp(const LinksyApp());
+}
+
+class LinksyApp extends StatelessWidget {
+  const LinksyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp.router(
+      title: 'Linksy',
+      debugShowCheckedModeBanner: false,
+      theme: AppTheme.light,
+      routerConfig: appRouter,
+    );
+  }
+}