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/dayDetail.js

@@ -28,8 +28,7 @@ let sheetScrollY = 0;
 function parseDataAttr(raw) {
   if (!raw) return [];
   try {
-    const s = raw.replace(/"/g, '"');
-    const parsed = JSON.parse(s);
+    const parsed = JSON.parse(raw);
     return Array.isArray(parsed) ? parsed : [];
   } catch (e) {
     return [];