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.

webapp/index.html

@@ -22,7 +22,7 @@
     <div class="error" id="error" hidden></div>
     <div class="access-denied" id="accessDenied" hidden>
       <p>Доступ запрещён.</p>
-      <p class="muted">Откройте календарь из Telegram.</p>
+      <p class="muted" id="accessDeniedHint">Откройте календарь из Telegram.</p>
       <p class="debug" id="accessDeniedDebug"></p>
     </div>
   </div>