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/api.test.js

@@ -0,0 +1,104 @@
+/**
+ * Unit tests for api: buildFetchOptions, fetchDuties (403 handling, AbortError).
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach, beforeAll } from "vitest";
+
+beforeAll(() => {
+  document.body.innerHTML =
+    '<div id="calendar"></div><h2 id="monthTitle"></h2>' +
+    '<div id="dutyList"></div><div id="loading"></div><div id="error"></div>' +
+    '<div id="accessDenied"></div><div class="header"></div><div class="weekdays"></div>' +
+    '<button id="prevMonth"></button><button id="nextMonth"></button>';
+});
+
+const mockGetInitData = vi.fn();
+vi.mock("./auth.js", () => ({ getInitData: () => mockGetInitData() }));
+
+import { buildFetchOptions, fetchDuties } from "./api.js";
+import { state } from "./dom.js";
+
+describe("buildFetchOptions", () => {
+  beforeEach(() => {
+    state.lang = "ru";
+  });
+
+  it("sets X-Telegram-Init-Data when initData provided", () => {
+    const opts = buildFetchOptions("init-data-string");
+    expect(opts.headers["X-Telegram-Init-Data"]).toBe("init-data-string");
+    opts.cleanup();
+  });
+
+  it("omits X-Telegram-Init-Data when initData empty", () => {
+    const opts = buildFetchOptions("");
+    expect(opts.headers["X-Telegram-Init-Data"]).toBeUndefined();
+    opts.cleanup();
+  });
+
+  it("sets Accept-Language from state.lang", () => {
+    state.lang = "en";
+    const opts = buildFetchOptions("");
+    expect(opts.headers["Accept-Language"]).toBe("en");
+    opts.cleanup();
+  });
+
+  it("returns signal and cleanup function", () => {
+    const opts = buildFetchOptions("");
+    expect(opts.signal).toBeDefined();
+    expect(typeof opts.cleanup).toBe("function");
+    opts.cleanup();
+  });
+
+  it("cleanup clears timeout and removes external abort listener", () => {
+    const controller = new AbortController();
+    const opts = buildFetchOptions("", controller.signal);
+    opts.cleanup();
+    controller.abort();
+  });
+});
+
+describe("fetchDuties", () => {
+  const originalFetch = globalThis.fetch;
+
+  beforeEach(() => {
+    mockGetInitData.mockReturnValue("test-init-data");
+    state.lang = "ru";
+  });
+
+  afterEach(() => {
+    globalThis.fetch = originalFetch;
+  });
+
+  it("returns JSON on 200", async () => {
+    globalThis.fetch = vi.fn().mockResolvedValue({
+      ok: true,
+      status: 200,
+      json: () => Promise.resolve([{ id: 1 }]),
+    });
+    const result = await fetchDuties("2025-02-01", "2025-02-28");
+    expect(result).toEqual([{ id: 1 }]);
+  });
+
+  it("throws ACCESS_DENIED on 403 with server detail from body", async () => {
+    globalThis.fetch = vi.fn().mockResolvedValue({
+      ok: false,
+      status: 403,
+      json: () => Promise.resolve({ detail: "Custom access denied" }),
+    });
+    await expect(fetchDuties("2025-02-01", "2025-02-28")).rejects.toMatchObject({
+      message: "ACCESS_DENIED",
+      serverDetail: "Custom access denied",
+    });
+  });
+
+  it("rethrows AbortError when request is aborted", async () => {
+    const aborter = new AbortController();
+    const abortError = new DOMException("aborted", "AbortError");
+    globalThis.fetch = vi.fn().mockImplementation(() => {
+      return Promise.reject(abortError);
+    });
+    await expect(
+      fetchDuties("2025-02-01", "2025-02-28", aborter.signal)
+    ).rejects.toMatchObject({ name: "AbortError" });
+  });
+});