feat: unify language handling across the application

- Updated the language configuration to use a single source of truth from `DEFAULT_LANGUAGE` for the bot, API, and Mini App, eliminating auto-detection from user settings.
- Refactored the `get_lang` function to always return `DEFAULT_LANGUAGE`, ensuring consistent language usage throughout the application.
- Modified the handling of language in various components, including API responses and UI elements, to reflect the new language management approach.
- Enhanced documentation and comments to clarify the changes in language handling.
- Added unit tests to verify the new language handling behavior and ensure coverage for the updated functionality.

webapp/js/i18n.js

@@ -1,9 +1,7 @@
 /**
- * Internationalization: language detection and translations for webapp.
+ * Internationalization: language from backend config (window.__DT_LANG) and translations.
  */
 
-import { getInitData } from "./auth.js";
-
 /** @type {Record<string, Record<string, string>>} */
 export const MESSAGES = {
   en: {
@@ -135,34 +133,25 @@ const WEEKDAY_KEYS = [
  * @param {string} code - e.g. 'ru', 'en', 'uk'
  * @returns {'ru'|'en'}
  */
-function normalizeLang(code) {
-  if (!code || typeof code !== "string") return "ru";
+export function normalizeLang(code) {
+  if (!code || typeof code !== "string") return "en";
   const lower = code.toLowerCase();
   if (lower.startsWith("ru")) return "ru";
   return "en";
 }
 
 /**
- * Detect language: Telegram initData user.language_code → navigator → fallback 'ru'.
+ * Get application language from backend config (window.__DT_LANG).
+ * Set by /app/config.js from DEFAULT_LANGUAGE. Fallback to 'en' if missing or invalid.
  * @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);
+  const raw =
+    typeof window !== "undefined" && window.__DT_LANG != null
+      ? String(window.__DT_LANG)
+      : "";
+  const lang = normalizeLang(raw);
+  return lang === "ru" ? "ru" : "en";
 }
 
 /**