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/settings/general/General.tsx

@@ -2251,6 +2251,10 @@ function Messages() {
   const [urlPreview, setUrlPreview] = useSetting(settingsAtom, 'urlPreview');
   const [encUrlPreview, setEncUrlPreview] = useSetting(settingsAtom, 'encUrlPreview');
   const [showHiddenEvents, setShowHiddenEvents] = useSetting(settingsAtom, 'showHiddenEvents');
+  const [enforceRetentionLocally, setEnforceRetentionLocally] = useSetting(
+    settingsAtom,
+    'enforceRetentionLocally',
+  );
 
   return (
     <Box direction="Column" gap="100">
@@ -2348,6 +2352,19 @@ function Messages() {
           }
         />
       </SequenceCard>
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Enforce Message Retention"
+          description="Permanently delete your own messages once a room's retention window (Room Settings → Message Retention) has passed. Off by default; only affects your own messages."
+          after={
+            <Switch
+              variant="Primary"
+              value={enforceRetentionLocally}
+              onChange={setEnforceRetentionLocally}
+            />
+          }
+        />
+      </SequenceCard>
     </Box>
   );
 }