feat(bills): link event label to its event detail page

The Anlass / event name on the invoice detail page and the bills list
now links through to /admin/events/:id when the invoice references a
real event row. The list link stops propagation so it doesn't trigger
the row's invoice navigation. Falls back to plain text when the invoice
carries only a free-text event snapshot. Customer portal unchanged
(no admin route access).

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

@@ -386,7 +386,14 @@ export const BillDetailPage: React.FC = () => {
       <Card>
         <div className="grid grid-cols-2 md:grid-cols-4 gap-4 text-sm">
           {inv.eventName && (
-            <div><div className="text-neutral-600 dark:text-neutral-300">{t('bills.field.eventName', 'Event')}</div><div>{inv.eventName}{inv.eventDate ? ` · ${inv.eventDate}` : ''}</div></div>
+            <div><div className="text-neutral-600 dark:text-neutral-300">{t('bills.field.eventName', 'Event')}</div>
+              <div>
+                {inv.eventId ? (
+                  <Link to={`/admin/events/${inv.eventId}`} className="text-theme hover:underline">{inv.eventName}</Link>
+                ) : inv.eventName}
+                {inv.eventDate ? ` · ${inv.eventDate}` : ''}
+              </div>
+            </div>
           )}
           <div><div className="text-neutral-600 dark:text-neutral-300">{t('bills.field.issueDate', 'Issued')}</div><div>{fmtDate(inv.issueDate)}</div></div>
           <div><div className="text-neutral-600 dark:text-neutral-300">{t('bills.field.dueDate', 'Due')}</div><div>{fmtDate(inv.dueDate)}</div></div>

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

@@ -167,7 +167,13 @@ export const BillsListPage: React.FC = () => {
                           )}
                         </td>
                         <td className="px-3 py-2">{inv.customer.companyName || inv.customer.displayName || inv.customer.email}</td>
-                        <td className="px-3 py-2 truncate max-w-xs">{inv.eventName || '—'}</td>
+                        <td className="px-3 py-2 truncate max-w-xs">
+                          {inv.eventName
+                            ? (inv.eventId
+                                ? <Link to={`/admin/events/${inv.eventId}`} className="text-theme hover:underline" onClick={(e) => e.stopPropagation()}>{inv.eventName}</Link>
+                                : inv.eventName)
+                            : '—'}
+                        </td>
                         <td className="px-3 py-2 text-xs text-muted-theme">
                           {inv.installmentTotal > 1 ? `${inv.installmentIndex + 1}/${inv.installmentTotal} · ${inv.installmentLabel || ''}` : '—'}
                         </td>