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/i18n.test.js

@@ -0,0 +1,87 @@
+/**
+ * Unit tests for i18n: getLang, t (fallback, params), monthName.
+ */
+
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+const mockGetInitData = vi.fn();
+vi.mock("./auth.js", () => ({ getInitData: () => mockGetInitData() }));
+
+import { getLang, t, monthName, MESSAGES } from "./i18n.js";
+
+describe("getLang", () => {
+  const origNavigator = globalThis.navigator;
+
+  beforeEach(() => {
+    mockGetInitData.mockReset();
+  });
+
+  it("returns lang from initData user when present", () => {
+    mockGetInitData.mockReturnValue(
+      "user=" + encodeURIComponent(JSON.stringify({ language_code: "en" }))
+    );
+    expect(getLang()).toBe("en");
+  });
+
+  it("normalizes ru from initData", () => {
+    mockGetInitData.mockReturnValue(
+      "user=" + encodeURIComponent(JSON.stringify({ language_code: "ru" }))
+    );
+    expect(getLang()).toBe("ru");
+  });
+
+  it("falls back to navigator.language when initData empty", () => {
+    mockGetInitData.mockReturnValue("");
+    Object.defineProperty(globalThis, "navigator", {
+      value: { ...origNavigator, language: "en-US", languages: ["en-US", "en"] },
+      configurable: true,
+    });
+    expect(getLang()).toBe("en");
+  });
+
+  it("normalizes to en for unknown language code", () => {
+    mockGetInitData.mockReturnValue(
+      "user=" + encodeURIComponent(JSON.stringify({ language_code: "uk" }))
+    );
+    expect(getLang()).toBe("en");
+  });
+});
+
+describe("t", () => {
+  it("returns translation for existing key", () => {
+    expect(t("en", "app.title")).toBe("Duty Calendar");
+    expect(t("ru", "app.title")).toBe("Календарь дежурств");
+  });
+
+  it("falls back to en when key missing in lang", () => {
+    expect(t("ru", "app.title")).toBe("Календарь дежурств");
+    expect(t("en", "loading")).toBe("Loading…");
+  });
+
+  it("returns key when key missing in both", () => {
+    expect(t("en", "missing.key")).toBe("missing.key");
+    expect(t("ru", "unknown")).toBe("unknown");
+  });
+
+  it("replaces params placeholder", () => {
+    expect(t("en", "duty.until", { time: "14:00" })).toBe("until 14:00");
+    expect(t("ru", "duty.until", { time: "09:30" })).toBe("до 09:30");
+  });
+
+  it("handles empty params", () => {
+    expect(t("en", "loading", {})).toBe("Loading…");
+  });
+});
+
+describe("monthName", () => {
+  it("returns month name for 0-based index", () => {
+    expect(monthName("en", 0)).toBe("January");
+    expect(monthName("en", 11)).toBe("December");
+    expect(monthName("ru", 0)).toBe("Январь");
+  });
+
+  it("returns empty string for out-of-range", () => {
+    expect(monthName("en", 12)).toBe("");
+    expect(monthName("en", -1)).toBe("");
+  });
+});