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/dateUtils.js

@@ -84,16 +84,6 @@ export function dateKeyToDDMM(key) {
   return key.slice(8, 10) + "." + key.slice(5, 7);
 }
 
-/**
- * Format ISO date as HH:MM in local time.
- * @param {string} isoStr - ISO date string
- * @returns {string} HH:MM
- */
-export function formatTimeLocal(isoStr) {
-  const d = new Date(isoStr);
-  return String(d.getHours()).padStart(2, "0") + ":" + String(d.getMinutes()).padStart(2, "0");
-}
-
 /**
  * Format ISO string as HH:MM (local).
  * @param {string} isoStr - ISO date string