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();

webapp/style.css

@@ -8,6 +8,17 @@
   --today: #bb9af7;
 }
 
+html {
+  scrollbar-gutter: stable;
+  scrollbar-width: none;
+  -ms-overflow-style: none;
+  overscroll-behavior: none;
+}
+
+html::-webkit-scrollbar {
+  display: none;
+}
+
 * {
   box-sizing: border-box;
 }
@@ -84,7 +95,7 @@ body {
 }
 
 .calendar-sticky.is-scrolled {
-  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.25);
+  box-shadow: 0 4px 6px -4px rgba(0, 0, 0, 0.25);
 }
 
 .calendar {