Enhance Telegram WebApp theme integration and update favicon

- Introduced a new function `applyThemeParamsToCss` to map Telegram theme parameters to CSS variables, improving theme customization.
- Updated the `applyTheme` function to utilize the new theme parameters and set background/header colors accordingly.
- Added a favicon to the project by linking `favicon.png` in `index.html`.
- Modified CSS for the light theme to use CSS variables for better flexibility and maintainability.
- Enhanced styling for light theme elements, including container borders and calendar sticky behavior.

webapp/style.css

@@ -11,14 +11,15 @@
   --error: #f7768e;
 }
 
+/* Light theme: prefer Telegram themeParams (--tg-theme-*), fallback to Telegram-like palette */
 [data-theme="light"] {
-  --bg: #d5d6db;
-  --surface: #b8b9c4;
-  --text: #343b58;
-  --muted: #6b7089;
-  --accent: #2e7de0;
+  --bg: var(--tg-theme-bg-color, #f0f1f3);
+  --surface: var(--tg-theme-secondary-bg-color, #e0e2e6);
+  --text: var(--tg-theme-text-color, #343b58);
+  --muted: var(--tg-theme-hint-color, #6b7089);
+  --accent: var(--tg-theme-link-color, #2e7de0);
   --duty: #587d0a;
-  --today: #7847b3;
+  --today: var(--tg-theme-link-color, var(--tg-theme-accent-text-color, #2481cc));
   --unavailable: #b8860b;
   --vacation: #0d6b9e;
   --error: #c43b3b;
@@ -56,6 +57,10 @@ body {
   padding-bottom: env(safe-area-inset-bottom, 12px);
 }
 
+[data-theme="light"] .container {
+  border-radius: 12px;
+}
+
 .header {
   display: flex;
   align-items: center;
@@ -114,6 +119,10 @@ body {
   box-shadow: 0 4px 6px -4px rgba(0, 0, 0, 0.25);
 }
 
+[data-theme="light"] .calendar-sticky.is-scrolled {
+  box-shadow: 0 4px 6px -4px rgba(0, 0, 0, 0.08);
+}
+
 .calendar {
   display: grid;
   grid-template-columns: repeat(7, 1fr);