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/features/common-settings/general/index.ts

@@ -5,6 +5,7 @@ export * from './RoomJoinRules';
 export * from './RoomProfile';
 export * from './RoomPublish';
 export * from './RoomQuality';
+export * from './RoomRetention';
 export * from './RoomShareInvite';
 export * from './RoomUpgrade';
 export * from './RoomVoiceLimit';