feat: add translation language setting & onboarding flow

- Add separate translation_language setting (independent from interface language)
- Implement 3-step onboarding for new users:
  1. Choose interface language
  2. Choose learning language
  3. Choose translation language
- Fix localization issues when using callback.message (user_id from state)
- Add UserService.get_user_by_id() method
- Add get_user_translation_lang() helper in i18n
- Update all handlers to use correct translation language
- Add localization keys for onboarding (ru/en/ja)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

bot/handlers/vocabulary.py

@@ -9,7 +9,7 @@ from database.models import WordSource
 from services.user_service import UserService
 from services.vocabulary_service import VocabularyService
 from services.ai_service import ai_service
-from utils.i18n import t, get_user_lang
+from utils.i18n import t, get_user_lang, get_user_translation_lang
 
 router = Router()
 
@@ -73,9 +73,9 @@ async def process_word_addition(message: Message, state: FSMContext, word: str):
     async with async_session_maker() as session:
         user = await UserService.get_user_by_telegram_id(session, message.from_user.id)
     source_lang = user.learning_language if user else 'en'
-    ui_lang = user.language_interface if user else 'ru'
+    translation_lang = get_user_translation_lang(user)
     word_data = await ai_service.translate_word_with_contexts(
-        word, source_lang=source_lang, translation_lang=ui_lang, max_translations=3
+        word, source_lang=source_lang, translation_lang=translation_lang, max_translations=3
     )
 
     # Удаляем сообщение о загрузке
@@ -141,7 +141,8 @@ async def confirm_add_word(callback: CallbackQuery, state: FSMContext):
         # Получаем пользователя для языков
         user = await UserService.get_user_by_telegram_id(session, callback.from_user.id)
         source_lang = user.learning_language if user else 'en'
-        ui_lang = user.language_interface if user else 'ru'
+        translation_lang = get_user_translation_lang(user)
+        ui_lang = get_user_lang(user)
 
         # Добавляем слово в базу
         new_word = await VocabularyService.add_word(
@@ -150,7 +151,7 @@ async def confirm_add_word(callback: CallbackQuery, state: FSMContext):
             word_original=word_data["word"],
             word_translation=word_data["translation"],
             source_lang=source_lang,
-            translation_lang=ui_lang,
+            translation_lang=translation_lang,
             transcription=word_data.get("transcription"),
             category=word_data.get("category"),
             difficulty_level=word_data.get("difficulty"),
@@ -168,7 +169,7 @@ async def confirm_add_word(callback: CallbackQuery, state: FSMContext):
 
         # Получаем общее количество слов
         words_count = await VocabularyService.get_words_count(session, user_id, learning_lang=user.learning_language)
-        lang = ui_lang or 'ru'
+        lang = ui_lang
 
     await callback.message.edit_text(
         t(lang, 'add.added_success', word=word_data['word'], count=words_count)