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/constants.js

@@ -7,14 +7,3 @@ export const RETRY_DELAY_MS = 800;
 export const RETRY_AFTER_ACCESS_DENIED_MS = 1200;
 
 export const THEME_BG = { dark: "#17212b", light: "#d5d6db" };
-
-export const MONTHS = [
-  "Январь", "Февраль", "Март", "Апрель", "Май", "Июнь",
-  "Июль", "Август", "Сентябрь", "Октябрь", "Ноябрь", "Декабрь"
-];
-
-export const EVENT_TYPE_LABELS = {
-  duty: "Дежурство",
-  unavailable: "Недоступен",
-  vacation: "Отпуск"
-};