feat: add Flutter web frontend with authentication and recipe management features

flutter/lib/features/auth/data/auth_providers.dart

@@ -0,0 +1,34 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import '../../../core/api/api_providers.dart';
+import '../../../core/platform/platform_providers.dart';
+import 'auth_repository.dart';
+
+final authRepositoryProvider = Provider<AuthRepository>((ref) {
+  return AuthRepository(
+    ref.watch(apiClientProvider),
+    ref.watch(tokenStorageProvider),
+  );
+});
+
+final authStateProvider = AsyncNotifierProvider<AuthNotifier, String?>(() {
+  return AuthNotifier();
+});
+
+class AuthNotifier extends AsyncNotifier<String?> {
+  @override
+  Future<String?> build() async {
+    return ref.watch(authRepositoryProvider).getToken();
+  }
+
+  Future<void> login(String email, String password) async {
+    state = const AsyncLoading();
+    state = await AsyncValue.guard(
+      () => ref.read(authRepositoryProvider).login(email, password),
+    );
+  }
+
+  Future<void> logout() async {
+    await ref.read(authRepositoryProvider).logout();
+    state = const AsyncData(null);
+  }
+}

flutter/lib/features/auth/data/auth_repository.dart

@@ -0,0 +1,28 @@
+import 'dart:convert';
+import '../../core/api/api_client.dart';
+import '../../core/platform/token_storage.dart';
+
+class AuthRepository {
+  final ApiClient _api;
+  final ITokenStorage _storage;
+
+  AuthRepository(this._api, this._storage);
+
+  Future<String> login(String email, String password) async {
+    final response = await _api.post(
+      '/api/auth/login',
+      jsonEncode({'email': email, 'password': password}),
+    );
+    if (response.statusCode != 200 && response.statusCode != 201) {
+      throw Exception('Login failed: ${response.statusCode}');
+    }
+    final data = jsonDecode(response.body) as Map<String, dynamic>;
+    final token = data['access_token'] as String;
+    await _storage.saveToken(token);
+    return token;
+  }
+
+  Future<void> logout() => _storage.deleteToken();
+
+  Future<String?> getToken() => _storage.getToken();
+}