chore: bootstrap flutter reader app skeleton

lib/features/auth/presentation/login_screen.dart

@@ -0,0 +1,19 @@
+import 'package:flutter/material.dart';
+
+import '../../../shared/widgets/feature_placeholder.dart';
+
+class LoginScreen extends StatelessWidget {
+  const LoginScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('Dang nhap')),
+      body: const FeaturePlaceholder(
+        title: 'Google Login',
+        description:
+            'Khung dang nhap Google OAuth cho mobile auth endpoint. Se bo sung token refresh va secure storage trong phase tiep theo.',
+      ),
+    );
+  }
+}