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/main.js

@@ -3,6 +3,7 @@
  */
 
 import { initTheme, applyTheme } from "./theme.js";
+import { getLang, t, weekdayLabels } from "./i18n.js";
 import { getInitData } from "./auth.js";
 import { isLocalhost } from "./auth.js";
 import { RETRY_DELAY_MS, RETRY_AFTER_ACCESS_DENIED_MS } from "./constants.js";
@@ -12,7 +13,8 @@ import {
   prevBtn,
   nextBtn,
   loadingEl,
-  errorEl
+  errorEl,
+  weekdaysEl
 } from "./dom.js";
 import { showAccessDenied, hideAccessDenied, showError, setNavEnabled } from "./ui.js";
 import { fetchDuties, fetchCalendarEvents } from "./api.js";
@@ -32,6 +34,20 @@ import {
 
 initTheme();
 
+state.lang = getLang();
+document.documentElement.lang = state.lang;
+document.title = t(state.lang, "app.title");
+if (loadingEl) loadingEl.textContent = t(state.lang, "loading");
+const dayLabels = weekdayLabels(state.lang);
+if (weekdaysEl) {
+  const spans = weekdaysEl.querySelectorAll("span");
+  spans.forEach((span, i) => {
+    if (dayLabels[i]) span.textContent = dayLabels[i];
+  });
+}
+if (prevBtn) prevBtn.setAttribute("aria-label", t(state.lang, "nav.prev_month"));
+if (nextBtn) nextBtn.setAttribute("aria-label", t(state.lang, "nav.next_month"));
+
 /**
  * Run callback when Telegram WebApp is ready (or immediately outside Telegram).
  * Expands and applies theme when in TWA.
@@ -79,12 +95,12 @@ function requireTelegramOrLocalhost(onAllowed) {
         onAllowed();
         return;
       }
-      showAccessDenied();
+      showAccessDenied(undefined);
       if (loadingEl) loadingEl.classList.add("hidden");
     }, RETRY_DELAY_MS);
     return;
   }
-  showAccessDenied();
+  showAccessDenied(undefined);
   if (loadingEl) loadingEl.classList.add("hidden");
 }
 
@@ -151,7 +167,7 @@ async function loadMonth() {
       }
       return;
     }
-    showError(e.message || "Не удалось загрузить данные.");
+    showError(e.message || t(state.lang, "error_generic"));
     setNavEnabled(true);
     return;
   }