feat: implement client-side localization infrastructure

- Add languageProvider (StateProvider<String>, default 'ru') with
  supportedLanguages map matching backend locale.Supported
- Wire Accept-Language header into AuthInterceptor via languageGetter
  callback; all API requests now carry the current language
- Sync language from user profile preferences into languageProvider
  on every ProfileNotifier load/update
- Add language field to UpdateProfileRequest, serialized as
  preferences.language in PUT /profile
- Profile screen: НАСТРОЙКИ section displays current language;
  edit sheet adds DropdownButtonFormField for language selection

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/core/api/api_client.dart

@@ -5,7 +5,11 @@ import 'auth_interceptor.dart';
 class ApiClient {
   late final Dio _dio;
 
-  ApiClient({required String baseUrl, required SecureStorageService storage}) {
+  ApiClient({
+    required String baseUrl,
+    required SecureStorageService storage,
+    required String Function() languageGetter,
+  }) {
     _dio = Dio(BaseOptions(
       baseUrl: baseUrl,
       connectTimeout: const Duration(seconds: 60),
@@ -14,7 +18,7 @@ class ApiClient {
     ));
 
     _dio.interceptors.addAll([
-      AuthInterceptor(storage: storage, dio: _dio),
+      AuthInterceptor(storage: storage, dio: _dio, languageGetter: languageGetter),
       LogInterceptor(requestBody: true, responseBody: true),
     ]);
   }

client/lib/core/api/auth_interceptor.dart

@@ -4,21 +4,28 @@ import '../auth/secure_storage.dart';
 class AuthInterceptor extends Interceptor {
   final SecureStorageService _storage;
   final Dio _dio;
+  final String Function() _languageGetter;
 
   // Prevents multiple simultaneous token refresh requests
   bool _isRefreshing = false;
   final List<({RequestOptions options, ErrorInterceptorHandler handler})>
       _pendingRequests = [];
 
-  AuthInterceptor({required SecureStorageService storage, required Dio dio})
-      : _storage = storage,
-        _dio = dio;
+  AuthInterceptor({
+    required SecureStorageService storage,
+    required Dio dio,
+    required String Function() languageGetter,
+  })  : _storage = storage,
+        _dio = dio,
+        _languageGetter = languageGetter;
 
   @override
   Future<void> onRequest(
     RequestOptions options,
     RequestInterceptorHandler handler,
   ) async {
+    options.headers['Accept-Language'] = _languageGetter();
+
     if (options.path.startsWith('/auth/')) {
       return handler.next(options);
     }