feat: implement internationalization for duty calendar

- Introduced a new `i18n.js` module for handling translations and language detection, supporting both Russian and English.
- Updated various components to utilize the new translation functions, enhancing user experience by providing localized messages for errors, hints, and UI elements.
- Refactored existing code in `api.js`, `calendar.js`, `dutyList.js`, `hints.js`, and `ui.js` to replace hardcoded strings with translated messages, improving maintainability and accessibility.
- Enhanced the initialization process in `main.js` to set the document language and update UI elements based on the user's language preference.

webapp/js/ui.js

@@ -3,6 +3,7 @@
  */
 
 import {
+  state,
   calendarEl,
   dutyListEl,
   loadingEl,
@@ -13,6 +14,7 @@ import {
   prevBtn,
   nextBtn
 } from "./dom.js";
+import { t } from "./i18n.js";
 
 /**
  * Show access-denied view and hide calendar/list/loading/error.
@@ -26,7 +28,8 @@ export function showAccessDenied(serverDetail) {
   if (loadingEl) loadingEl.classList.add("hidden");
   if (errorEl) errorEl.hidden = true;
   if (accessDeniedEl) {
-    accessDeniedEl.innerHTML = "<p>Доступ запрещён.</p>";
+    const msg = t(state.lang, "access_denied");
+    accessDeniedEl.innerHTML = "<p>" + msg + "</p>";
     if (serverDetail && serverDetail.trim()) {
       const detail = document.createElement("p");
       detail.className = "access-denied-detail";