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/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),
   );
 });