Enhance Telegram bot functionality and improve error handling

- Introduced a new function to set the default menu button for the Telegram bot's Web App.
- Updated the initData validation process to provide detailed error messages for authorization failures.
- Refactored the validate_init_data function to return both username and reason for validation failure.
- Enhanced the web application to handle access denial more gracefully, providing users with hints on how to access the calendar.
- Improved the README with additional instructions for configuring the bot's menu button and Web App URL.
- Updated tests to reflect changes in the validation process and error handling.

handlers/errors.py

@@ -8,7 +8,7 @@ from telegram.ext import ContextTypes
 logger = logging.getLogger(__name__)
 
 
-async def error_handler(update: object, context: ContextTypes.DEFAULT_TYPE) -> None:
+async def error_handler(update: Update | None, context: ContextTypes.DEFAULT_TYPE) -> None:
     logger.exception("Exception while handling an update")
     if isinstance(update, Update) and update.effective_message:
         await update.effective_message.reply_text("Произошла ошибка. Попробуйте позже.")