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/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');