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/api.js

@@ -4,15 +4,18 @@
 
 import { FETCH_TIMEOUT_MS } from "./constants.js";
 import { getInitData } from "./auth.js";
+import { state } from "./dom.js";
+import { t } from "./i18n.js";
 
 /**
- * Build fetch options with init data header and timeout abort.
+ * Build fetch options with init data header, Accept-Language and timeout abort.
  * @param {string} initData - Telegram init data
  * @returns {{ headers: object, signal: AbortSignal, timeoutId: number }}
  */
 export function buildFetchOptions(initData) {
   const headers = {};
   if (initData) headers["X-Telegram-Init-Data"] = initData;
+  headers["Accept-Language"] = state.lang || "ru";
   const controller = new AbortController();
   const timeoutId = setTimeout(() => controller.abort(), FETCH_TIMEOUT_MS);
   return { headers, signal: controller.signal, timeoutId };
@@ -37,7 +40,7 @@ export async function fetchDuties(from, to) {
   try {
     const res = await fetch(url, { headers: opts.headers, signal: opts.signal });
     if (res.status === 403) {
-      let detail = "Доступ запрещён";
+      let detail = t(state.lang, "access_denied");
       try {
         const body = await res.json();
         if (body && body.detail !== undefined) {
@@ -53,11 +56,11 @@ export async function fetchDuties(from, to) {
       err.serverDetail = detail;
       throw err;
     }
-    if (!res.ok) throw new Error("Ошибка загрузки");
+    if (!res.ok) throw new Error(t(state.lang, "error_load_failed"));
     return res.json();
   } catch (e) {
     if (e.name === "AbortError") {
-      throw new Error("Не удалось загрузить данные. Проверьте интернет.");
+      throw new Error(t(state.lang, "error_network"));
     }
     throw e;
   } finally {