feat: show each account's last sign-in in Users

A right-aligned column at the end of every Users & roles row says how long ago the
account signed in, with the exact moment as the hint and as a line in the account
panel; a dash where nothing is recorded, and the demo account says it is not recorded.
formatDateTime formats that moment in the UI language rather than the browser's.

frontend/src/lib/format.ts

@@ -148,6 +148,12 @@ export function formatCountOrDash(n: number | null | undefined): string {
   return n != null ? formatViews(n) : "—";
 }
 
+// Absolute date AND time in the UI language (e.g. "16 Sept 2026, 08:30:14"). A bare `toLocaleString()`
+// formats in the browser's locale instead, so an English UI showed a Hungarian-shaped date. Empty for null.
+export function formatDateTime(iso: string | null | undefined): string {
+  return iso ? new Date(iso).toLocaleString(i18n.language) : "";
+}
+
 // Absolute day-precision date (e.g. "12 Jan 2017"), shown alongside a relative time. Empty for null.
 export function formatDate(iso: string | null | undefined, lang: string): string {
   return iso