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');

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,
+  ),
 );

client/lib/features/profile/profile_screen.dart

@@ -3,6 +3,7 @@ import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../../core/auth/auth_provider.dart';
+import '../../core/locale/language_provider.dart';
 import '../../core/theme/app_colors.dart';
 import '../../shared/models/user.dart';
 import 'profile_provider.dart';
@@ -114,6 +115,18 @@ class _ProfileBody extends StatelessWidget {
             ),
           ),
         ],
+        const SizedBox(height: 16),
+
+        // Settings
+        _SectionLabel('НАСТРОЙКИ'),
+        const SizedBox(height: 6),
+        _InfoCard(children: [
+          _InfoRow(
+            'Язык',
+            supportedLanguages[user.preferences['language'] as String? ?? 'ru'] ??
+                'Русский',
+          ),
+        ]),
         const SizedBox(height: 32),
 
         _LogoutButton(),
@@ -327,6 +340,7 @@ class _EditProfileSheetState extends ConsumerState<EditProfileSheet> {
   String? _gender;
   String? _goal;
   String? _activity;
+  String? _language;
   bool _saving = false;
 
   @override
@@ -341,6 +355,7 @@ class _EditProfileSheetState extends ConsumerState<EditProfileSheet> {
     _gender = u.gender;
     _goal = u.goal;
     _activity = u.activity;
+    _language = u.preferences['language'] as String? ?? 'ru';
   }
 
   @override
@@ -367,6 +382,7 @@ class _EditProfileSheetState extends ConsumerState<EditProfileSheet> {
       gender: _gender,
       goal: _goal,
       activity: _activity,
+      language: _language,
     );
 
     final ok = await ref.read(profileProvider.notifier).update(req);
@@ -555,6 +571,21 @@ class _EditProfileSheetState extends ConsumerState<EditProfileSheet> {
                       onSelectionChanged: (s) => setState(
                           () => _activity = s.isEmpty ? null : s.first),
                     ),
+                    const SizedBox(height: 20),
+
+                    // Language
+                    DropdownButtonFormField<String>(
+                      value: _language,
+                      decoration:
+                          const InputDecoration(labelText: 'Язык интерфейса'),
+                      items: supportedLanguages.entries
+                          .map((e) => DropdownMenuItem(
+                                value: e.key,
+                                child: Text(e.value),
+                              ))
+                          .toList(),
+                      onChanged: (v) => setState(() => _language = v),
+                    ),
                     const SizedBox(height: 32),
 
                     // Save

client/lib/features/profile/profile_service.dart

@@ -16,6 +16,7 @@ class UpdateProfileRequest {
   final String? gender;
   final String? activity;
   final String? goal;
+  final String? language;
 
   const UpdateProfileRequest({
     this.name,
@@ -25,6 +26,7 @@ class UpdateProfileRequest {
     this.gender,
     this.activity,
     this.goal,
+    this.language,
   });
 
   Map<String, dynamic> toJson() {
@@ -36,6 +38,7 @@ class UpdateProfileRequest {
     if (gender != null) map['gender'] = gender;
     if (activity != null) map['activity'] = activity;
     if (goal != null) map['goal'] = goal;
+    if (language != null) map['preferences'] = {'language': language};
     return map;
   }
 }