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_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