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/tasks.py

@@ -37,7 +37,7 @@ async def cmd_task(message: Message, state: FSMContext):
         )
 
         if not tasks:
-            await message.answer(t(user.level.value and (user.language_interface or 'ru') or 'ru', 'tasks.no_words'))
+            await message.answer(t(user.language_interface or 'ru', 'tasks.no_words'))
             return
 
         # Сохраняем задания в состоянии