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

flutter/lib/core/platform/platform_providers.dart

@@ -0,0 +1,9 @@
+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

@@ -0,0 +1,7 @@
+/// 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

@@ -0,0 +1,26 @@
+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);
+  }
+}