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/dateUtils.test.js

@@ -0,0 +1,159 @@
+/**
+ * Unit tests for dateUtils: localDateString, dutyOverlapsLocalDay,
+ * dutyOverlapsLocalRange, getMonday, formatHHMM.
+ */
+
+import { describe, it, expect } from "vitest";
+import {
+  localDateString,
+  dutyOverlapsLocalDay,
+  dutyOverlapsLocalRange,
+  getMonday,
+  formatHHMM,
+} from "./dateUtils.js";
+
+describe("localDateString", () => {
+  it("formats date as YYYY-MM-DD", () => {
+    const d = new Date(2025, 0, 15);
+    expect(localDateString(d)).toBe("2025-01-15");
+  });
+
+  it("pads month and day with zero", () => {
+    expect(localDateString(new Date(2025, 0, 5))).toBe("2025-01-05");
+    expect(localDateString(new Date(2025, 8, 9))).toBe("2025-09-09");
+  });
+
+  it("handles December and year boundary", () => {
+    expect(localDateString(new Date(2024, 11, 31))).toBe("2024-12-31");
+  });
+});
+
+describe("dutyOverlapsLocalDay", () => {
+  it("returns true when duty spans the whole day", () => {
+    const d = {
+      start_at: "2025-02-25T00:00:00Z",
+      end_at: "2025-02-25T23:59:59Z",
+    };
+    expect(dutyOverlapsLocalDay(d, "2025-02-25")).toBe(true);
+  });
+
+  it("returns true when duty overlaps part of the day", () => {
+    const d = {
+      start_at: "2025-02-25T09:00:00Z",
+      end_at: "2025-02-25T14:00:00Z",
+    };
+    expect(dutyOverlapsLocalDay(d, "2025-02-25")).toBe(true);
+  });
+
+  it("returns true when duty continues from previous day", () => {
+    const d = {
+      start_at: "2025-02-24T22:00:00Z",
+      end_at: "2025-02-25T06:00:00Z",
+    };
+    expect(dutyOverlapsLocalDay(d, "2025-02-25")).toBe(true);
+  });
+
+  it("returns false when duty ends before the day", () => {
+    const d = {
+      start_at: "2025-02-24T09:00:00Z",
+      end_at: "2025-02-24T18:00:00Z",
+    };
+    expect(dutyOverlapsLocalDay(d, "2025-02-25")).toBe(false);
+  });
+
+  it("returns false when duty starts after the day", () => {
+    const d = {
+      start_at: "2025-02-26T09:00:00Z",
+      end_at: "2025-02-26T18:00:00Z",
+    };
+    expect(dutyOverlapsLocalDay(d, "2025-02-25")).toBe(false);
+  });
+});
+
+describe("dutyOverlapsLocalRange", () => {
+  it("returns true when duty overlaps the range", () => {
+    const d = {
+      start_at: "2025-02-24T12:00:00Z",
+      end_at: "2025-02-26T12:00:00Z",
+    };
+    expect(dutyOverlapsLocalRange(d, "2025-02-25", "2025-02-28")).toBe(true);
+  });
+
+  it("returns true when duty is entirely inside the range", () => {
+    const d = {
+      start_at: "2025-02-26T09:00:00Z",
+      end_at: "2025-02-26T18:00:00Z",
+    };
+    expect(dutyOverlapsLocalRange(d, "2025-02-25", "2025-02-28")).toBe(true);
+  });
+
+  it("returns false when duty ends before range start", () => {
+    const d = {
+      start_at: "2025-02-20T09:00:00Z",
+      end_at: "2025-02-22T18:00:00Z",
+    };
+    expect(dutyOverlapsLocalRange(d, "2025-02-25", "2025-02-28")).toBe(false);
+  });
+
+  it("returns false when duty starts after range end", () => {
+    const d = {
+      start_at: "2025-03-01T09:00:00Z",
+      end_at: "2025-03-01T18:00:00Z",
+    };
+    expect(dutyOverlapsLocalRange(d, "2025-02-25", "2025-02-28")).toBe(false);
+  });
+});
+
+describe("getMonday", () => {
+  it("returns same day when date is Monday", () => {
+    const monday = new Date(2025, 0, 6); // 6 Jan 2025 is Monday
+    const result = getMonday(monday);
+    expect(result.getFullYear()).toBe(2025);
+    expect(result.getMonth()).toBe(0);
+    expect(result.getDate()).toBe(6);
+    expect(result.getDay()).toBe(1);
+  });
+
+  it("returns previous Monday for Wednesday", () => {
+    const wed = new Date(2025, 0, 8);
+    const result = getMonday(wed);
+    expect(result.getDay()).toBe(1);
+    expect(result.getDate()).toBe(6);
+  });
+
+  it("returns Monday of same week for Sunday", () => {
+    const sun = new Date(2025, 0, 12);
+    const result = getMonday(sun);
+    expect(result.getDay()).toBe(1);
+    expect(result.getDate()).toBe(6);
+  });
+});
+
+describe("formatHHMM", () => {
+  it("formats ISO string as HH:MM in local time", () => {
+    const s = "2025-02-25T14:30:00Z";
+    const result = formatHHMM(s);
+    expect(result).toMatch(/^\d{2}:\d{2}$/);
+    const d = new Date(s);
+    const expected = (d.getHours() < 10 ? "0" : "") + d.getHours() + ":" + (d.getMinutes() < 10 ? "0" : "") + d.getMinutes();
+    expect(result).toBe(expected);
+  });
+
+  it("returns empty string for null", () => {
+    expect(formatHHMM(null)).toBe("");
+  });
+
+  it("returns empty string for undefined", () => {
+    expect(formatHHMM(undefined)).toBe("");
+  });
+
+  it("returns empty string for empty string", () => {
+    expect(formatHHMM("")).toBe("");
+  });
+
+  it("pads hours and minutes with zero", () => {
+    const s = "2025-02-25T09:05:00Z";
+    const result = formatHHMM(s);
+    expect(result).toMatch(/^\d{2}:\d{2}$/);
+  });
+});