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/features/profile/profile_provider.dart

@@ -1,32 +1,48 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../../core/locale/language_provider.dart';
 import '../../shared/models/user.dart';
 import 'profile_service.dart';
 
 class ProfileNotifier extends StateNotifier<AsyncValue<User>> {
   final ProfileService _service;
+  final void Function(String) _setLanguage;
 
-  ProfileNotifier(this._service) : super(const AsyncValue.loading()) {
+  ProfileNotifier(this._service, this._setLanguage)
+      : super(const AsyncValue.loading()) {
     load();
   }
 
   Future<void> load() async {
     state = const AsyncValue.loading();
     state = await AsyncValue.guard(() => _service.getProfile());
+    _syncLanguage();
   }
 
   Future<bool> update(UpdateProfileRequest req) async {
     try {
       final updated = await _service.updateProfile(req);
       state = AsyncValue.data(updated);
+      _syncLanguage();
       return true;
     } catch (_) {
       return false;
     }
   }
+
+  // Propagates the user's preferred language to the global languageProvider.
+  void _syncLanguage() {
+    final lang = state.valueOrNull?.preferences['language'] as String?;
+    if (lang != null && supportedLanguages.containsKey(lang)) {
+      _setLanguage(lang);
+    }
+  }
 }
 
 final profileProvider =
     StateNotifierProvider<ProfileNotifier, AsyncValue<User>>(
-  (ref) => ProfileNotifier(ref.read(profileServiceProvider)),
+  (ref) => ProfileNotifier(
+    ref.read(profileServiceProvider),
+    (lang) => ref.read(languageProvider.notifier).state = lang,
+  ),
 );