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/state/settings.ts

@@ -183,6 +183,9 @@ export interface Settings {
   urlPreview: boolean;
   encUrlPreview: boolean;
   showHiddenEvents: boolean;
+  // [MSC1763] Opt-in: permanently redact your OWN messages once a room's
+  // retention window passes (default off — nothing auto-deletes by surprise).
+  enforceRetentionLocally: boolean;
   legacyUsernameColor: boolean;
 
   showNotifications: boolean;
@@ -288,6 +291,7 @@ const defaultSettings: Settings = {
   urlPreview: true,
   encUrlPreview: true,
   showHiddenEvents: false,
+  enforceRetentionLocally: false,
   legacyUsernameColor: false,
 
   showNotifications: true,