Emit document title on error pages (404/500) per TZ 4.1.21

Previously the 404/500 React ErrorPage set the page content but not
document.title, so the browser tab showed the URL path instead of
a localized title. Added <title> element + imperative document.title
assignment (pattern from SeoHead.tsx) so both SSR and client set
the tab title to "<code> — <localized-title>", e.g. "404 — Страница
не найдена".

src/ui/errors/ErrorPage.tsx

@@ -120,9 +120,19 @@ export function ErrorPage({ code }: ErrorPageProps): JSX.Element {
   const support = translations?.support ?? "Поддержка";
   const refresh = translations?.refresh ?? (config.refreshKey ? "Обновить страницу" : undefined);
   const displayCode = code ?? "?";
+  const documentTitle = `${displayCode} — ${title}`;
+
+  // React 18 doesn't hoist <title> inside body to document.head, so set
+  // document.title imperatively on the client once translations resolve.
+  useEffect(() => {
+    if (typeof document !== "undefined" && translations) {
+      document.title = documentTitle;
+    }
+  }, [documentTitle, translations]);
 
   return (
     <>
+      <title>{documentTitle}</title>
       {/* noindex: error pages must not be indexed (TZ §4.1.21) */}
       <meta name="robots" content="noindex,nofollow" />
       <div className="error-page" data-testid={`error-page-${code ?? "unknown"}`}>