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/style.css

@@ -545,7 +545,7 @@ body.day-detail-sheet-open {
 
 .duty-marker {
   color: var(--duty);
-  background: rgba(158, 206, 106, 0.25);
+  background: color-mix(in srgb, var(--duty) 25%, transparent);
 }
 
 .unavailable-marker {
@@ -767,10 +767,6 @@ body.day-detail-sheet-open {
   border-left-color: var(--vacation);
 }
 
-[data-theme="dark"] .duty-marker {
-  background: color-mix(in srgb, var(--duty) 25%, transparent);
-}
-
 .duty-item .duty-item-type {
   grid-column: 1;
   grid-row: 1;