refactor: restructure web application with modular JavaScript and remove legacy code

- Deleted the `app.js` file and migrated its functionality to a modular structure with multiple JavaScript files for better organization and maintainability.
- Updated `index.html` to reference the new `main.js` module, ensuring proper loading of the application.
- Introduced new utility modules for API requests, authentication, calendar handling, and DOM manipulation to enhance code clarity and separation of concerns.
- Enhanced CSS styles for improved layout and theming consistency across the application.
- Added comprehensive comments and documentation to new modules to facilitate future development and understanding.

webapp/index.html

@@ -28,6 +28,6 @@
     </div>
   </div>
   <script src="https://telegram.org/js/telegram-web-app.js"></script>
-  <script src="app.js"></script>
+  <script type="module" src="js/main.js"></script>
 </body>
 </html>