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/dutyList.js

@@ -10,7 +10,7 @@ import {
   firstDayOfMonth,
   lastDayOfMonth,
   dateKeyToDDMM,
-  formatTimeLocal,
+  formatHHMM,
   formatDateKey
 } from "./dateUtils.js";
 
@@ -25,8 +25,8 @@ export function dutyTimelineCardHtml(d, isCurrent) {
   const endLocal = localDateString(new Date(d.end_at));
   const startDDMM = dateKeyToDDMM(startLocal);
   const endDDMM = dateKeyToDDMM(endLocal);
-  const startTime = formatTimeLocal(d.start_at);
-  const endTime = formatTimeLocal(d.end_at);
+  const startTime = formatHHMM(d.start_at);
+  const endTime = formatHHMM(d.end_at);
   let timeStr;
   if (startLocal === endLocal) {
     timeStr = startDDMM + ", " + startTime + " – " + endTime;
@@ -69,14 +69,14 @@ export function dutyItemHtml(d, typeLabelOverride, showUntilEnd, extraClass) {
   if (extraClass) itemClass += " " + extraClass;
   let timeOrRange = "";
   if (showUntilEnd && d.event_type === "duty") {
-    timeOrRange = t(lang, "duty.until", { time: formatTimeLocal(d.end_at) });
+    timeOrRange = t(lang, "duty.until", { time: formatHHMM(d.end_at) });
   } else if (d.event_type === "vacation" || d.event_type === "unavailable") {
     const startStr = formatDateKey(d.start_at);
     const endStr = formatDateKey(d.end_at);
     timeOrRange = startStr === endStr ? startStr : startStr + " – " + endStr;
   } else {
     timeOrRange =
-      formatTimeLocal(d.start_at) + " – " + formatTimeLocal(d.end_at);
+      formatHHMM(d.start_at) + " – " + formatHHMM(d.end_at);
   }
   return (
     '<div class="' +