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.

config.py

@@ -1,7 +1,6 @@
 """Load configuration from environment. Fail fast if BOT_TOKEN is missing."""
 
 import os
-from pathlib import Path
 
 from dotenv import load_dotenv
 
@@ -16,7 +15,6 @@ if not BOT_TOKEN:
 DATABASE_URL = os.getenv("DATABASE_URL", "sqlite:///data/duty_teller.db")
 MINI_APP_BASE_URL = os.getenv("MINI_APP_BASE_URL", "").rstrip("/")
 HTTP_PORT = int(os.getenv("HTTP_PORT", "8080"))
-DATA_DIR = Path(__file__).resolve().parent / "data"
 
 # Miniapp access: comma-separated Telegram usernames (no @). Empty = no one allowed.
 _raw_allowed = os.getenv("ALLOWED_USERNAMES", "").strip()