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);
     }

client/lib/core/auth/auth_provider.dart

@@ -1,12 +1,13 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:firebase_auth/firebase_auth.dart' as fb;
 
 import '../../shared/models/user.dart';
 import '../api/api_client.dart';
 import '../config/app_config.dart';
+import '../locale/language_provider.dart';
 import 'auth_service.dart';
 import 'secure_storage.dart';
-import 'package:firebase_auth/firebase_auth.dart' as fb;
 
 enum AuthStatus { unknown, authenticated, unauthenticated }
 
@@ -144,6 +145,7 @@ final apiClientProvider = Provider<ApiClient>((ref) {
   return ApiClient(
     baseUrl: config.apiBaseUrl,
     storage: storage,
+    languageGetter: () => ref.read(languageProvider),
   );
 });
 

client/lib/core/locale/language_provider.dart

@@ -0,0 +1,23 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+/// Supported ISO 639-1 language codes with their native names.
+/// Must match the backend locale.Supported map.
+const supportedLanguages = <String, String>{
+  'en': 'English',
+  'ru': 'Русский',
+  'es': 'Español',
+  'de': 'Deutsch',
+  'fr': 'Français',
+  'it': 'Italiano',
+  'pt': 'Português',
+  'zh': '中文',
+  'ja': '日本語',
+  'ko': '한국어',
+  'ar': 'العربية',
+  'hi': 'हिन्दी',
+};
+
+/// Current app language (ISO 639-1 code).
+/// Synced from user.preferences['language'] after the profile loads or is updated.
+/// Defaults to 'ru'.
+final languageProvider = StateProvider<String>((_) => 'ru');