feat: update language support and enhance API functionality

- Changed the default language in `index.html` from Russian to English, updating the title and button aria-labels for improved accessibility.
- Refactored the `buildFetchOptions` function in `api.js` to include an optional external abort signal, enhancing request management.
- Updated `fetchDuties` and `fetchCalendarEvents` to support request cancellation using the new abort signal, improving error handling.
- Added unit tests for the API functions to ensure proper functionality, including handling of 403 errors and request cancellations.
- Enhanced CSS styles for duty markers to improve visual consistency.
- Removed unused code and improved the overall structure of the JavaScript files for better maintainability.

webapp/js/utils.js

@@ -2,13 +2,19 @@
  * Common utilities.
  */
 
+const ESCAPE_MAP = {
+  "&": "&",
+  "<": "&lt;",
+  ">": "&gt;",
+  '"': "&quot;",
+  "'": "&#39;",
+};
+
 /**
  * Escape string for safe use in HTML (text content / attributes).
  * @param {string} s - Raw string
  * @returns {string} HTML-escaped string
  */
 export function escapeHtml(s) {
-  const div = document.createElement("div");
-  div.textContent = s;
-  return div.innerHTML;
+  return String(s).replace(/[&<>"']/g, (c) => ESCAPE_MAP[c]);
 }