feat(tasks): completed tasks are deleted after a delay (default 30 days), "Erledigte leeren"

thoughts.completed_at is set by triggers on every way a task is completed (migration 14; tasks
completed earlier count from the update). Users choose the delay (7/30/90/365 days or never,
users.done_retention_days); for tasks in a list the owner's delay counts. An hourly job deletes
expired completed tasks (not in the demo) and tells their viewers. POST /tasks/clear-done clears
completed tasks the user may edit. The done tab shows the setting and the button. v0.35.0.

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

frontend/src/features/triage/TasksView.tsx

@@ -690,6 +690,31 @@ export const TasksView: React.FC<TasksViewProps> = ({
     onRefreshStats();
   };
 
+  // Completed tasks: cleared by hand or deleted after the owner's delay (done_cleanup.go)
+  const [doneRetentionDays, setDoneRetentionDays] = useState<number>(currentUser?.doneRetentionDays ?? 30);
+  useEffect(() => {
+    if (currentUser?.doneRetentionDays !== undefined) setDoneRetentionDays(currentUser.doneRetentionDays);
+  }, [currentUser?.doneRetentionDays]);
+
+  const handleChangeDoneRetention = async (days: number) => {
+    const previous = doneRetentionDays;
+    setDoneRetentionDays(days);
+    try {
+      await api.updateSettings({ doneRetentionDays: days });
+    } catch {
+      setDoneRetentionDays(previous);
+    }
+  };
+
+  const handleClearDone = async () => {
+    const scope = selectedListId === 'all' ? 'aller Listen, die du bearbeiten darfst,' : `aus „${currentSelectedList?.title || 'dieser Liste'}“`;
+    if (!window.confirm(`Alle erledigten Aufgaben ${scope} endgültig löschen?`)) return;
+    triggerHaptic(20);
+    await api.clearDoneTasks(selectedListId === 'all' ? '' : selectedListId);
+    await loadData();
+    onRefreshStats();
+  };
+
   const handlePurgeAllDiscarded = async () => {
     if (!window.confirm('Alle verworfenen Gedanken endgültig löschen?')) return;
     triggerHaptic(20);
@@ -2586,8 +2611,44 @@ export const TasksView: React.FC<TasksViewProps> = ({
                 Papierkorb leeren
               </button>
             )}
+            {filter === 'done' && doneTasks.length > 0 && !isSelectedListReadOnly && (
+              <button onClick={handleClearDone} className="btn-action min-h-[38px] sm:min-h-0 ml-auto">
+                <Trash2 className="w-3.5 h-3.5" />
+                Erledigte leeren
+              </button>
+            )}
           </div>
 
+          {/* What happens to completed tasks; the delay of the list owner counts */}
+          {filter === 'done' && (
+            <div className="flex flex-wrap items-center gap-x-2 gap-y-1 text-[11px] text-muted px-1">
+              {currentSelectedList && currentSelectedList.isOwner === false ? (
+                <span>
+                  Erledigte Aufgaben dieser Liste räumt die Einstellung von {currentSelectedList.ownerName || 'der Eigentümerin bzw. dem Eigentümer'} auf.
+                </span>
+              ) : (
+                <>
+                  <span>Erledigte Aufgaben löschen</span>
+                  <select
+                    value={doneRetentionDays}
+                    onChange={(e) => handleChangeDoneRetention(Number(e.target.value))}
+                    className="h-9 sm:h-6 px-1.5 input-field text-xs bg-panel"
+                    aria-label="Erledigte Aufgaben löschen nach"
+                  >
+                    <option value={7}>nach 7 Tagen</option>
+                    <option value={30}>nach 30 Tagen</option>
+                    <option value={90}>nach 90 Tagen</option>
+                    <option value={365}>nach einem Jahr</option>
+                    <option value={0}>nie</option>
+                  </select>
+                  <span className="text-faint">
+                    {doneRetentionDays > 0 ? '(gezählt ab dem Abhaken; gilt für deine eigenen Listen)' : '(bleiben, bis du sie leerst)'}
+                  </span>
+                </>
+              )}
+            </div>
+          )}
+
           {loading ? (
             <div className="p-6 text-center text-faint text-sm">Lade Aufgaben…</div>
           ) : filter === 'discarded' ? (