fix(retention): a tiny or brand-new retention value can't wipe a room (#298)

m.room.retention can be set to anything by anyone with state power. A
fat-fingered `max_lifetime: 1000` blanked the timeline for every Lotus
user and, for members with "Enforce Message Retention" on, the sweeper
redacted all their messages within a minute. The RETENTION_MIN_MS floor
existed but was never applied.

- effectiveRetentionMs(): anything under 10 min counts as off; used by
  getRoomRetentionMs (sweeper) and the timeline's hiding.
- The sweeper never deletes under a policy less than 24 h old, so a
  mistake or a malicious change can be reverted before anything is lost.
  (Local hiding of expired messages still applies at once.)

Verified headless (the issue's repro, enforcement on): after
max_lifetime 1000, all 4 messages stay visible and none are redacted
after 45 s (before: timeline blank, all 4 redacted). Unit tests for the
floor and the grace window.

Fixes #298

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/features/room/RoomTimeline.tsx

@@ -121,7 +121,7 @@ import { ThreadSummary } from './thread/ThreadSummary';
 import { usePowerLevelsContext } from '../../hooks/usePowerLevels';
 import { GetContentCallback, MessageEvent, StateEvent } from '../../../types/matrix/room';
 import { useStateEvent } from '../../hooks/useStateEvent';
-import { RetentionContent, isExpired } from '../../utils/retention';
+import { RetentionContent, effectiveRetentionMs, isExpired } from '../../utils/retention';
 import { useKeyDown } from '../../hooks/useKeyDown';
 import { useDocumentFocusChange } from '../../hooks/useDocumentFocusChange';
 import { RenderMessageContent } from '../../components/RenderMessageContent';
@@ -494,7 +494,10 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
   // timeline (unless "show hidden events" is on). Reactive so a policy change
   // re-renders. `undefined` = no policy.
   const retentionEvent = useStateEvent(room, StateEvent.RoomRetention);
-  const retentionMs = retentionEvent?.getContent<RetentionContent>().max_lifetime;
+  // [Gitea #298] Same floor as everywhere else: a tiny value must not blank the room.
+  const retentionMs = effectiveRetentionMs(
+    retentionEvent?.getContent<RetentionContent>().max_lifetime,
+  );
   const [showDeveloperTools] = useSetting(settingsAtom, 'developerTools');
   const [lotusTerminal] = useSetting(settingsAtom, 'lotusTerminal');