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/app.js

@@ -24,15 +24,41 @@
     return "light";
   }
 
+  /**
+   * Map Telegram themeParams (snake_case) to CSS variables (--tg-theme-*).
+   * Only set when Telegram WebApp and themeParams are available.
+   */
+  function applyThemeParamsToCss() {
+    var twa = window.Telegram?.WebApp;
+    var params = twa?.themeParams;
+    if (!params) return;
+    var root = document.documentElement;
+    var map = [
+      ["bg_color", "bg-color"],
+      ["secondary_bg_color", "secondary-bg-color"],
+      ["text_color", "text-color"],
+      ["hint_color", "hint-color"],
+      ["link_color", "link-color"],
+      ["button_color", "button-color"],
+      ["header_bg_color", "header-bg-color"],
+      ["accent_text_color", "accent-text-color"]
+    ];
+    map.forEach(function (pair) {
+      var value = params[pair[0]];
+      if (value) root.style.setProperty("--tg-theme-" + pair[1], value);
+    });
+  }
+
   function applyTheme() {
     var scheme = getTheme();
     document.documentElement.dataset.theme = scheme;
-    var bg = THEME_BG[scheme] || THEME_BG.dark;
-    if (window.Telegram?.WebApp?.setBackgroundColor) {
-      window.Telegram.WebApp.setBackgroundColor(bg);
-    }
-    if (window.Telegram?.WebApp?.setHeaderColor) {
-      window.Telegram.WebApp.setHeaderColor(bg);
+    var twa = window.Telegram?.WebApp;
+    if (twa) {
+      if (twa.setBackgroundColor) twa.setBackgroundColor("bg_color");
+      if (twa.setHeaderColor) twa.setHeaderColor("bg_color");
+      applyThemeParamsToCss();
+    } else {
+      /* Outside Telegram: only data-theme is set; no setBackgroundColor/setHeaderColor. */
     }
   }