fix: dates follow the UI language, not the browser's

Seven sites formatted with a bare toLocaleString()/toLocaleDateString(), so an
English UI on a Hungarian machine printed Hungarian-shaped dates (and the reverse).
They now go through formatDateTime/formatDate, which take the i18n language.

frontend/src/components/About.tsx

@@ -4,13 +4,14 @@ import { useTranslation } from "react-i18next";
 import { Info, Sparkles } from "lucide-react";
 import { api } from "../lib/api";
 import { FRONTEND_VERSION, FRONTEND_SHA, FRONTEND_BUILD_DATE } from "../lib/version";
+import { formatDateTime } from "../lib/format";
 import { Button } from "./ui/Button";
 import Modal from "./ui/Modal";
 
 function fmtDate(d?: string | null): string {
   if (!d) return "—";
   const t = new Date(d);
-  return isNaN(t.getTime()) ? d : t.toLocaleString();
+  return isNaN(t.getTime()) ? d : formatDateTime(d);
 }
 
 // About dialog: app + build + schema versions (frontend/backend/database), plus a