fix: sync html lang attribute with active locale (#5753)

Co-authored-by: memoclaw <265580040+memoclaw@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

web/src/utils/i18n.ts

@@ -96,6 +96,7 @@ export const loadLocale = (locale: string): Locale => {
   const validLocale = isValidLocale(locale) ? (locale as Locale) : findNearestMatchedLanguage(navigator.language);
   setStoredLocale(validLocale);
   i18n.changeLanguage(validLocale);
+  document.documentElement.lang = validLocale;
   return validLocale;
 };