feat: JLPT levels for Japanese, custom practice scenarios, UI improvements

- Add separate level systems: CEFR (A1-C2) for European languages, JLPT (N5-N1) for Japanese
- Store levels per language in new `levels_by_language` JSON field
- Add custom scenario option in AI practice mode
- Show action buttons after practice ends (new dialogue, tasks, words)
- Fix level display across all handlers to use correct level system
- Add Alembic migration for levels_by_language field
- Update all locale files (ru, en, ja) with new keys

🤖 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
+from utils.i18n import t, get_user_lang
 
 router = Router()
 
@@ -58,10 +58,8 @@ async def process_word_addition(message: Message, state: FSMContext, word: str):
         # Проверяем, есть ли уже такое слово
         existing_word = await VocabularyService.find_word(session, user.id, word)
         if existing_word:
-            await message.answer(
-                f"Слово '<b>{word}</b>' уже есть в твоём словаре!\n"
-                f"Перевод: {existing_word.word_translation}"
-            )
+            lang = get_user_lang(user)
+            await message.answer(t(lang, 'add.exists', word=word, translation=existing_word.word_translation))
             await state.clear()
             return