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/room-settings/general/General.tsx

@@ -12,6 +12,7 @@ import {
   RoomPublishedAddresses,
   RoomPublish,
   RoomQuality,
+  RoomRetention,
   RoomShareInvite,
   RoomUpgrade,
   RoomVoiceLimit,
@@ -56,6 +57,10 @@ export function General({ requestClose }: GeneralProps) {
                 <RoomEncryption permissions={permissions} />
                 <RoomPublish permissions={permissions} />
               </Box>
+              <Box direction="Column" gap="100">
+                <Text size="L400">Message Retention</Text>
+                <RoomRetention permissions={permissions} />
+              </Box>
               <Box direction="Column" gap="100">
                 <Text size="L400">Voice</Text>
                 <RoomVoiceLimit permissions={permissions} />