Enhance Telegram WebApp integration and update CSS styles

- Added functionality to set background and header colors for the Telegram WebApp to improve visual consistency.
- Updated CSS to hide scrollbars and enhance the overall user experience by preventing overscroll behavior.
- Adjusted box-shadow for the calendar sticky header for a more refined appearance.

webapp/app.js

@@ -448,6 +448,13 @@
       if (window.Telegram.WebApp.expand) {
         window.Telegram.WebApp.expand();
       }
+      var bg = "#1a1b26";
+      if (window.Telegram.WebApp.setBackgroundColor) {
+        window.Telegram.WebApp.setBackgroundColor(bg);
+      }
+      if (window.Telegram.WebApp.setHeaderColor) {
+        window.Telegram.WebApp.setHeaderColor(bg);
+      }
       setTimeout(cb, 0);
     } else {
       cb();