feat(rooms): Disappearing Messages (MSC1763 m.room.retention)

B1 of the Matrix protocol-gaps roadmap, gate-green (688 tests):
- StateEvent.RoomRetention + a shared utils/retention.ts (presets, isExpired,
  getRoomRetentionMs) with tests.
- RoomRetention settings control (PL-gated preset buttons Off/1d/1w/1m) in Room
  Settings → General → Message Retention.
- Timeline hides events past the room's max_lifetime (gated behind Show Hidden
  Events, like redactions) — messages visually disappear, losslessly.
- Opt-in setting enforceRetentionLocally (default OFF) + a headless
  RetentionSweeper that permanently redacts the user's OWN expired messages
  (own-only, loaded-timeline scope, dedupe + retry). Nothing auto-deletes unless
  the user opts in.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -48,6 +48,7 @@ import { STATUS_EXPIRY_KEY, STATUS_MSG_KEY } from '../../features/settings/accou
 import { useDeepLinkNavigate } from '../../hooks/useDeepLinkNavigate';
 import { toastQueueAtom } from '../../state/toast';
 import { useReminders } from '../../hooks/useReminders';
+import { getRoomRetentionMs, isExpired } from '../../utils/retention';
 import { useTauriUpdater } from '../../hooks/useTauriUpdater';
 import { TauriDesktopFeatures } from '../../components/TauriDesktopFeatures';
 import { KeyboardShortcutsDialog, useKeyboardShortcutsTrigger } from '../../features/shortcuts';
@@ -687,6 +688,62 @@ function ReminderMonitor() {
   return null;
 }
 
+// MSC1763: opt-in local enforcement of room retention. When enabled, permanently
+// redacts the user's OWN messages once a room's retention window passes. Own-only
+// (no redact PL needed); scoped to loaded live-timeline events; dedupes in-flight
+// redactions and retries on the next tick. Default-off, so nothing auto-deletes
+// unless the user turns it on.
+function RetentionSweeper() {
+  const mx = useMatrixClient();
+  const [enforceRetentionLocally] = useSetting(settingsAtom, 'enforceRetentionLocally');
+  const enabledRef = useRef(enforceRetentionLocally);
+  enabledRef.current = enforceRetentionLocally;
+  const redactingRef = useRef<Set<string>>(new Set());
+
+  useEffect(() => {
+    const check = () => {
+      if (!enabledRef.current) return;
+      const myId = mx.getUserId();
+      if (!myId) return;
+      const now = Date.now();
+      mx.getRooms().forEach((room) => {
+        const maxLifetime = getRoomRetentionMs(room);
+        if (!maxLifetime) return;
+        room
+          .getLiveTimeline()
+          .getEvents()
+          .forEach((ev) => {
+            const evId = ev.getId();
+            if (!evId || ev.getSender() !== myId) return;
+            if (ev.isState() || ev.isRedacted() || ev.isSending()) return;
+            const t = ev.getType();
+            // Only actual messages — never our membership/topic/reactions.
+            if (t !== 'm.room.message' && t !== 'm.room.encrypted' && t !== 'm.sticker') return;
+            if (!isExpired(ev.getTs(), maxLifetime, now)) return;
+            if (redactingRef.current.has(evId)) return;
+            redactingRef.current.add(evId);
+            mx.redactEvent(room.roomId, evId, undefined, { reason: 'expired' }).catch(() => {
+              redactingRef.current.delete(evId);
+            });
+          });
+      });
+    };
+
+    check();
+    const interval = setInterval(check, 30_000);
+    const onVisible = () => {
+      if (document.visibilityState === 'visible') check();
+    };
+    document.addEventListener('visibilitychange', onVisible);
+    return () => {
+      clearInterval(interval);
+      document.removeEventListener('visibilitychange', onVisible);
+    };
+  }, [mx]);
+
+  return null;
+}
+
 const TAURI_UPDATE_CHECK_INTERVAL = 12 * 60 * 60_000; // 12 hours
 const TAURI_UPDATE_LAST_CHECK_KEY = 'lotus.tauriUpdateLastCheck';
 
@@ -773,6 +830,7 @@ export function ClientNonUIFeatures({ children }: ClientNonUIFeaturesProps) {
       <InviteNotifications />
       <MessageNotifications />
       <ReminderMonitor />
+      <RetentionSweeper />
       <TauriUpdateFeature />
       <TauriDesktopFeatures />
       <LotusDenoiseFeature />