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/utils.test.js

@@ -0,0 +1,42 @@
+/**
+ * Unit tests for escapeHtml edge cases.
+ */
+
+import { describe, it, expect } from "vitest";
+import { escapeHtml } from "./utils.js";
+
+describe("escapeHtml", () => {
+  it("escapes ampersand", () => {
+    expect(escapeHtml("a & b")).toBe("a & b");
+  });
+
+  it("escapes less-than and greater-than", () => {
+    expect(escapeHtml("<script>")).toBe("&lt;script&gt;");
+  });
+
+  it("escapes double quote", () => {
+    expect(escapeHtml('say "hello"')).toBe("say &quot;hello&quot;");
+  });
+
+  it("escapes single quote", () => {
+    expect(escapeHtml("it's")).toBe("it&#39;s");
+  });
+
+  it("escapes all special chars together", () => {
+    expect(escapeHtml('&<>"\'')).toBe("&amp;&lt;&gt;&quot;&#39;");
+  });
+
+  it("returns unchanged string when no special chars", () => {
+    expect(escapeHtml("plain text")).toBe("plain text");
+  });
+
+  it("handles empty string", () => {
+    expect(escapeHtml("")).toBe("");
+  });
+
+  it("coerces non-string to string", () => {
+    expect(escapeHtml(123)).toBe("123");
+    expect(escapeHtml(null)).toBe("null");
+    expect(escapeHtml(undefined)).toBe("undefined");
+  });
+});