feat(P6B.T9): PIN-Idle-Auto-Lock setting + idle watcher

Adds opt-in (default OFF) auto-lock: after X minutes of no user input
the app calls signOut() (full memory wipe + PIN re-entry on next open).
Settings dropdown (Aus / 5 / 15 / 30 / 60 min) lives in SecurityCenter
below the existing wipe-on-close toggle. The idle timer is mounted in
AppShell via useIdleAutoLock; activity events are throttled to 1 Hz to
avoid timer thrash on rapid mouse movement. The localStorage key is
added to PRESERVE_LOCAL_STORAGE so a wipe never silently disables the
feature.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/desktop/src/components/SecurityCenter.tsx

@@ -1,5 +1,11 @@
 import { useState } from 'react';
 
+import {
+  type AutoLockMinutes,
+  getAutoLockMinutes,
+  notifyAutoLockChanged,
+  setAutoLockMinutes,
+} from '../lib/autoLockSettings';
 import { isWipeOnCloseEnabled, setWipeOnClose } from '../lib/memoryWipeSettings';
 import {
   changePin,
@@ -21,6 +27,7 @@ export function SecurityCenter({ userId }: Props) {
   const [recovery, setRecovery] = useState<string | null>(null);
   const [migration, setMigration] = useState<LegacyMigrationReport | null>(null);
   const [wipeOnClose, setWipeOnCloseState] = useState<boolean>(() => isWipeOnCloseEnabled());
+  const [autoLockMinutes, setAutoLockMinutesState] = useState<AutoLockMinutes>(() => getAutoLockMinutes());
 
   async function handleRetryMigration() {
     setBusy(true); setMsg(null); setMigration(null);
@@ -149,6 +156,39 @@ export function SecurityCenter({ userId }: Props) {
         </label>
       </section>
 
+      <section>
+        <h3 className="mb-2 text-xs font-medium uppercase tracking-wide text-fg-muted">
+          Auto-Lock nach Inaktivität
+        </h3>
+        <p className="mb-2 text-xs text-fg-muted">
+          Verlangt erneute PIN-Eingabe nach der gewählten Inaktivitätsdauer. Empfohlen für gemeinsam genutzte Rechner.
+        </p>
+        <div className="flex items-center justify-between gap-3">
+          <div>
+            <div className="text-sm font-medium text-fg">Automatisch sperren</div>
+            <div className="text-xs text-fg-muted">
+              Verlangt erneute PIN-Eingabe nach X Minuten Inaktivität.
+            </div>
+          </div>
+          <select
+            value={autoLockMinutes}
+            onChange={(e) => {
+              const next = Number(e.target.value) as AutoLockMinutes;
+              setAutoLockMinutes(next);
+              notifyAutoLockChanged(next);
+              setAutoLockMinutesState(next);
+            }}
+            className="cursor-pointer rounded-md border border-line bg-surface-2 px-3 py-1.5 text-sm text-fg focus:border-accent focus:outline-none focus:ring-1 focus:ring-accent/40"
+          >
+            <option value={0}>Aus</option>
+            <option value={5}>5 min</option>
+            <option value={15}>15 min</option>
+            <option value={30}>30 min</option>
+            <option value={60}>60 min</option>
+          </select>
+        </div>
+      </section>
+
       <section>
         <h3 className="mb-2 text-xs font-medium uppercase tracking-wide text-rose-400">Identität zurücksetzen</h3>
         <p className="mb-2 text-xs text-fg-muted">Erstellt einen neuen Schlüssel. Alle alten Chats werden unlesbar.</p>