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/words.py

@@ -10,6 +10,7 @@ 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.levels import get_user_level_for_language
 
 router = Router()
 
@@ -50,9 +51,10 @@ async def cmd_words(message: Message, state: FSMContext):
         generating_msg = await message.answer(t(lang, 'words.generating', theme=theme))
 
         # Генерируем слова через AI
+        current_level = get_user_level_for_language(user)
         words = await ai_service.generate_thematic_words(
             theme=theme,
-            level=user.level.value,
+            level=current_level,
             count=10,
             learning_lang=user.learning_language,
             translation_lang=user.language_interface,
@@ -69,7 +71,7 @@ async def cmd_words(message: Message, state: FSMContext):
             theme=theme,
             words=words,
             user_id=user.id,
-            level=user.level.name
+            level=current_level
         )
         await state.set_state(WordsStates.viewing_words)