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/dom.js

@@ -39,5 +39,7 @@ export const state = {
   /** @type {object[]} */
   lastDutiesForList: [],
   /** @type {ReturnType<typeof setInterval>|null} */
-  todayRefreshInterval: null
+  todayRefreshInterval: null,
+  /** @type {'ru'|'en'} */
+  lang: "ru"
 };