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/i18n.js

@@ -0,0 +1,180 @@
+/**
+ * Internationalization: language detection and translations for webapp.
+ */
+
+import { getInitData } from "./auth.js";
+
+/** @type {Record<string, Record<string, string>>} */
+export const MESSAGES = {
+  en: {
+    "app.title": "Duty Calendar",
+    loading: "Loading…",
+    access_denied: "Access denied.",
+    error_load_failed: "Load failed",
+    error_network: "Could not load data. Check your connection.",
+    error_generic: "Could not load data.",
+    "nav.prev_month": "Previous month",
+    "nav.next_month": "Next month",
+    "weekdays.mon": "Mon",
+    "weekdays.tue": "Tue",
+    "weekdays.wed": "Wed",
+    "weekdays.thu": "Thu",
+    "weekdays.fri": "Fri",
+    "weekdays.sat": "Sat",
+    "weekdays.sun": "Sun",
+    "month.jan": "January",
+    "month.feb": "February",
+    "month.mar": "March",
+    "month.apr": "April",
+    "month.may": "May",
+    "month.jun": "June",
+    "month.jul": "July",
+    "month.aug": "August",
+    "month.sep": "September",
+    "month.oct": "October",
+    "month.nov": "November",
+    "month.dec": "December",
+    "aria.duty": "On duty",
+    "aria.unavailable": "Unavailable",
+    "aria.vacation": "Vacation",
+    "aria.day_info": "Day info",
+    "event_type.duty": "Duty",
+    "event_type.unavailable": "Unavailable",
+    "event_type.vacation": "Vacation",
+    "duty.now_on_duty": "On duty now",
+    "duty.none_this_month": "No duties this month.",
+    "duty.today": "Today",
+    "duty.until": "until {time}",
+    "hint.from": "from",
+    "hint.to": "until",
+    "hint.duty_title": "Duty:",
+    "hint.events": "Events:"
+  },
+  ru: {
+    "app.title": "Календарь дежурств",
+    loading: "Загрузка…",
+    access_denied: "Доступ запрещён.",
+    error_load_failed: "Ошибка загрузки",
+    error_network: "Не удалось загрузить данные. Проверьте интернет.",
+    error_generic: "Не удалось загрузить данные.",
+    "nav.prev_month": "Предыдущий месяц",
+    "nav.next_month": "Следующий месяц",
+    "weekdays.mon": "Пн",
+    "weekdays.tue": "Вт",
+    "weekdays.wed": "Ср",
+    "weekdays.thu": "Чт",
+    "weekdays.fri": "Пт",
+    "weekdays.sat": "Сб",
+    "weekdays.sun": "Вс",
+    "month.jan": "Январь",
+    "month.feb": "Февраль",
+    "month.mar": "Март",
+    "month.apr": "Апрель",
+    "month.may": "Май",
+    "month.jun": "Июнь",
+    "month.jul": "Июль",
+    "month.aug": "Август",
+    "month.sep": "Сентябрь",
+    "month.oct": "Октябрь",
+    "month.nov": "Ноябрь",
+    "month.dec": "Декабрь",
+    "aria.duty": "Дежурные",
+    "aria.unavailable": "Недоступен",
+    "aria.vacation": "Отпуск",
+    "aria.day_info": "Информация о дне",
+    "event_type.duty": "Дежурство",
+    "event_type.unavailable": "Недоступен",
+    "event_type.vacation": "Отпуск",
+    "duty.now_on_duty": "Сейчас дежурит",
+    "duty.none_this_month": "В этом месяце дежурств нет.",
+    "duty.today": "Сегодня",
+    "duty.until": "до {time}",
+    "hint.from": "с",
+    "hint.to": "до",
+    "hint.duty_title": "Дежурство:",
+    "hint.events": "События:"
+  }
+};
+
+const MONTH_KEYS = [
+  "month.jan", "month.feb", "month.mar", "month.apr", "month.may", "month.jun",
+  "month.jul", "month.aug", "month.sep", "month.oct", "month.nov", "month.dec"
+];
+
+const WEEKDAY_KEYS = [
+  "weekdays.mon", "weekdays.tue", "weekdays.wed", "weekdays.thu",
+  "weekdays.fri", "weekdays.sat", "weekdays.sun"
+];
+
+/**
+ * Normalize language code to 'ru' or 'en'.
+ * @param {string} code - e.g. 'ru', 'en', 'uk'
+ * @returns {'ru'|'en'}
+ */
+function normalizeLang(code) {
+  if (!code || typeof code !== "string") return "ru";
+  const lower = code.toLowerCase();
+  if (lower.startsWith("ru")) return "ru";
+  return "en";
+}
+
+/**
+ * Detect language: Telegram initData user.language_code → navigator → fallback 'ru'.
+ * @returns {'ru'|'en'}
+ */
+export function getLang() {
+  const initData = getInitData();
+  if (initData) {
+    try {
+      const params = new URLSearchParams(initData);
+      const userStr = params.get("user");
+      if (userStr) {
+        const user = JSON.parse(decodeURIComponent(userStr));
+        const code = user && user.language_code;
+        if (code) return normalizeLang(code);
+      }
+    } catch (e) {
+      /* ignore */
+    }
+  }
+  const nav = navigator.language || (navigator.languages && navigator.languages[0]) || "";
+  return normalizeLang(nav);
+}
+
+/**
+ * Get translated string; fallback to en if key missing in lang. Supports {placeholder}.
+ * @param {'ru'|'en'} lang
+ * @param {string} key - e.g. 'app.title', 'duty.until'
+ * @param {Record<string, string>} [params] - e.g. { time: '14:00' }
+ * @returns {string}
+ */
+export function t(lang, key, params = {}) {
+  const dict = MESSAGES[lang] || MESSAGES.en;
+  let s = dict[key];
+  if (s === undefined) s = MESSAGES.en[key];
+  if (s === undefined) return key;
+  Object.keys(params).forEach((k) => {
+    s = s.replace(new RegExp("\\{" + k + "\\}", "g"), params[k]);
+  });
+  return s;
+}
+
+/**
+ * Get month name by 0-based index.
+ * @param {'ru'|'en'} lang
+ * @param {number} month - 0–11
+ * @returns {string}
+ */
+export function monthName(lang, month) {
+  const key = MONTH_KEYS[month];
+  return key ? t(lang, key) : "";
+}
+
+/**
+ * Get weekday short labels (Mon–Sun order) for given lang.
+ * @param {'ru'|'en'} lang
+ * @returns {string[]}
+ */
+export function weekdayLabels(lang) {
+  return WEEKDAY_KEYS.map((k) => t(lang, k));
+}