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

flutter/lib/core/api/api_client.dart

@@ -0,0 +1,37 @@
+import 'dart:io';
+import 'package:http/http.dart' as http;
+
+/// Platform-neutral HTTP client wrapping the internal API base URL.
+/// Base URL is read from the FLUTTER_API_URL_INTERNAL environment variable
+/// (set by Docker) or falls back to localhost for local development.
+class ApiClient {
+  final String baseUrl;
+  final http.Client _client;
+
+  ApiClient({http.Client? client})
+      : baseUrl = const String.fromEnvironment(
+          'API_BASE_URL',
+          defaultValue: 'http://localhost:8080',
+        ),
+        _client = client ?? http.Client();
+
+  Map<String, String> _headers({String? token}) => {
+        'Content-Type': 'application/json',
+        if (token != null) 'Authorization': 'Bearer $token',
+      };
+
+  Future<http.Response> get(String path, {String? token}) =>
+      _client.get(Uri.parse('$baseUrl$path'), headers: _headers(token: token));
+
+  Future<http.Response> post(String path, String body, {String? token}) =>
+      _client.post(Uri.parse('$baseUrl$path'),
+          headers: _headers(token: token), body: body);
+
+  Future<http.Response> put(String path, String body, {String? token}) =>
+      _client.put(Uri.parse('$baseUrl$path'),
+          headers: _headers(token: token), body: body);
+
+  Future<http.Response> delete(String path, {String? token}) =>
+      _client.delete(Uri.parse('$baseUrl$path'),
+          headers: _headers(token: token));
+}

flutter/lib/core/api/api_providers.dart

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+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'api_client.dart';
+
+final apiClientProvider = Provider<ApiClient>((ref) => ApiClient());

flutter/lib/core/platform/platform_providers.dart

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+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'token_storage.dart';
+import 'web_token_storage.dart';
+
+/// Bind ITokenStorage to the web adapter.
+/// For mobile: swap WebTokenStorage for SecureTokenStorage here.
+final tokenStorageProvider = Provider<ITokenStorage>((ref) {
+  return WebTokenStorage();
+});

flutter/lib/core/platform/token_storage.dart

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+/// Platform-neutral contract for token storage.
+/// Web implementation uses SharedPreferences; mobile uses flutter_secure_storage.
+abstract interface class ITokenStorage {
+  Future<String?> getToken();
+  Future<void> saveToken(String token);
+  Future<void> deleteToken();
+}

flutter/lib/core/platform/web_token_storage.dart

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+import 'package:shared_preferences/shared_preferences.dart';
+import 'token_storage.dart';
+
+/// Web-adapter: stores JWT in SharedPreferences (localStorage on web).
+/// Replace with flutter_secure_storage adapter for Android/iOS.
+class WebTokenStorage implements ITokenStorage {
+  static const _key = 'auth_token';
+
+  @override
+  Future<String?> getToken() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString(_key);
+  }
+
+  @override
+  Future<void> saveToken(String token) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_key, token);
+  }
+
+  @override
+  Future<void> deleteToken() async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.remove(_key);
+  }
+}

flutter/lib/core/router/app_router.dart

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+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
+import '../../features/recipes/presentation/recipes_screen.dart';
+import '../../features/auth/presentation/login_screen.dart';
+
+final appRouterProvider = Provider<GoRouter>((ref) {
+  return GoRouter(
+    initialLocation: '/recipes',
+    routes: [
+      GoRoute(
+        path: '/login',
+        builder: (context, state) => const LoginScreen(),
+      ),
+      GoRoute(
+        path: '/recipes',
+        builder: (context, state) => const RecipesScreen(),
+      ),
+    ],
+  );
+});