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,
+  ),
 );

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;
   }
 }