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_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;
   }
 }