fix(bills): localize the event_date on the invoice detail card

The Anlass field rendered inv.eventDate verbatim (raw ISO from pg
date-as-Date serialization) while every other date on the card went
through useLocalizedDate. Route it through fmtDate so it honors the
general_date_format setting. (The event_id → /admin/events linkify was
already in place.)

frontend/src/pages/admin/bills/BillDetailPage.tsx

@@ -391,7 +391,7 @@ export const BillDetailPage: React.FC = () => {
                 {inv.eventId ? (
                   <Link to={`/admin/events/${inv.eventId}`} className="text-theme hover:underline">{inv.eventName}</Link>
                 ) : inv.eventName}
-                {inv.eventDate ? ` · ${inv.eventDate}` : ''}
+                {inv.eventDate ? ` · ${fmtDate(inv.eventDate)}` : ''}
               </div>
             </div>
           )}