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/dom.js

@@ -41,5 +41,13 @@ export const state = {
   /** @type {ReturnType<typeof setInterval>|null} */
   todayRefreshInterval: null,
   /** @type {'ru'|'en'} */
-  lang: "ru"
+  lang: "ru",
+  /** One-time bind flag for sticky scroll shadow listener. */
+  stickyScrollBound: false,
+  /** One-time bind flag for calendar (info button) hint document listeners. */
+  calendarHintBound: false,
+  /** One-time bind flag for duty marker hint document listeners. */
+  dutyMarkerHintBound: false,
+  /** Whether initData retry after ACCESS_DENIED has been attempted. */
+  initDataRetried: false
 };