feat: task trash, triage trash and retention settings for every area; admin can forbid keeping things for good

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/due.ts

@@ -48,3 +48,10 @@ export const formatDue = (due: string | undefined, now: Date = new Date()): stri
   else label = `${pad(d)}.${pad(m)}.${y}`;
   return time ? `${label}, ${time}` : label;
 };
+
+/** When something went to a trash, e.g. "01.11.2026, 14:05". */
+export const formatDeletedAt = (iso: string) => {
+  const d = new Date(iso);
+  if (Number.isNaN(d.getTime())) return '';
+  return d.toLocaleString('de-DE', { day: '2-digit', month: '2-digit', year: 'numeric', hour: '2-digit', minute: '2-digit' });
+};