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>

utils/i18n.py

@@ -31,6 +31,11 @@ def _resolve_key(data: Dict[str, Any], dotted_key: str) -> Any:
     return cur
 
 
+def get_user_lang(user) -> str:
+    """Унифицированное получение языка интерфейса пользователя."""
+    return (getattr(user, 'language_interface', None) if user else None) or 'ru'
+
+
 def t(lang: str, key: str, **kwargs) -> str:
     """Translate key for given lang; fallback to ru and to key itself.