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/calendar.js

@@ -29,6 +29,9 @@ export function calendarEventsByDate(events) {
   return byDate;
 }
 
+/** Max days to iterate per duty; prevents infinite loop on corrupted API data (end_at < start_at). */
+const MAX_DAYS_PER_DUTY = 366;
+
 /**
  * Group duties by local date (start_at/end_at are UTC).
  * @param {object[]} duties - Duties with start_at, end_at
@@ -39,14 +42,17 @@ export function dutiesByDate(duties) {
   duties.forEach((d) => {
     const start = new Date(d.start_at);
     const end = new Date(d.end_at);
+    if (end < start) return;
     const endLocal = localDateString(end);
-    let t = new Date(start);
-    while (true) {
-      const key = localDateString(t);
+    let cursor = new Date(start);
+    let iterations = 0;
+    while (iterations <= MAX_DAYS_PER_DUTY) {
+      const key = localDateString(cursor);
       if (!byDate[key]) byDate[key] = [];
       byDate[key].push(d);
       if (key === endLocal) break;
-      t.setDate(t.getDate() + 1);
+      cursor.setDate(cursor.getDate() + 1);
+      iterations++;
     }
   });
   return byDate;
@@ -104,14 +110,8 @@ export function renderCalendar(
         start_at: x.start_at,
         end_at: x.end_at
       }));
-      cell.setAttribute(
-        "data-day-duties",
-        JSON.stringify(dayPayload).replace(/"/g, "&quot;")
-      );
-      cell.setAttribute(
-        "data-day-events",
-        JSON.stringify(eventSummaries).replace(/"/g, "&quot;")
-      );
+      cell.setAttribute("data-day-duties", JSON.stringify(dayPayload));
+      cell.setAttribute("data-day-events", JSON.stringify(eventSummaries));
     }
 
     const ariaParts = [];