feat: restructure menu and add file import

- Consolidate "Add word" menu with submenu (Manual, Thematic, Import)
- Add file import support (.txt, .md) with AI batch translation
- Add vocabulary pagination with navigation buttons
- Add "Add word" button in tasks for new words mode
- Fix undefined variables bug in vocabulary confirm handler
- Add localization keys for add_menu in ru/en/ja

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

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

database/models.py

@@ -45,6 +45,7 @@ class WordSource(str, enum.Enum):
     CONTEXT = "context"  # Из контекста диалога
     IMPORT = "import"  # Импорт из текста
     ERROR = "error"  # Из ошибок в заданиях
+    AI_TASK = "ai_task"  # Из AI-задания
 
 
 class User(Base):