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.

main.py

@@ -1,8 +1,10 @@
 """Single entry point: build Application, run HTTP server + polling. Migrations run in Docker entrypoint."""
 
 import asyncio
+import json
 import logging
 import threading
+import urllib.request
 
 import config
 from telegram.ext import ApplicationBuilder
@@ -16,6 +18,36 @@ logging.basicConfig(
 logger = logging.getLogger(__name__)
 
 
+def _set_default_menu_button_webapp() -> None:
+    """Set the bot's default menu button to Web App so Telegram sends tgWebAppData when users open the app from the menu."""
+    if not (config.MINI_APP_BASE_URL and config.BOT_TOKEN):
+        return
+    menu_url = (config.MINI_APP_BASE_URL.rstrip("/") + "/app/").strip()
+    if not menu_url.startswith("https://"):
+        return
+    payload = {
+        "menu_button": {
+            "type": "web_app",
+            "text": "Календарь",
+            "web_app": {"url": menu_url},
+        }
+    }
+    req = urllib.request.Request(
+        f"https://api.telegram.org/bot{config.BOT_TOKEN}/setChatMenuButton",
+        data=json.dumps(payload).encode(),
+        headers={"Content-Type": "application/json"},
+        method="POST",
+    )
+    try:
+        with urllib.request.urlopen(req, timeout=10) as resp:
+            if resp.status == 200:
+                logger.info("Default menu button set to Web App: %s", menu_url)
+            else:
+                logger.warning("setChatMenuButton returned %s", resp.status)
+    except Exception as e:
+        logger.warning("Could not set menu button: %s", e)
+
+
 def _run_uvicorn(web_app, port: int) -> None:
     """Run uvicorn in a dedicated thread with its own event loop."""
     import uvicorn
@@ -29,6 +61,7 @@ def _run_uvicorn(web_app, port: int) -> None:
 
 
 def main() -> None:
+    _set_default_menu_button_webapp()
     app = ApplicationBuilder().token(config.BOT_TOKEN).build()
     register_handlers(app)