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/pages/client/ClientNonUIFeatures.tsx

@@ -68,7 +68,12 @@ import { setPresenceWithRetry } from '../../utils/presenceWrite';
 import { useDeepLinkNavigate } from '../../hooks/useDeepLinkNavigate';
 import { dismissToastAtom, toastQueueAtom } from '../../state/toast';
 import { useReminders } from '../../hooks/useReminders';
-import { getRoomRetentionMs, isExpired } from '../../utils/retention';
+import {
+  getRoomRetentionMs,
+  getRoomRetentionSetAt,
+  isExpired,
+  retentionDeletesAllowed,
+} from '../../utils/retention';
 import { useTauriUpdateProgress, useTauriUpdater } from '../../hooks/useTauriUpdater';
 import { settingsRequestAtom } from '../../state/settingsRequest';
 import { isNetworkUpdateError } from '../../utils/updateErrors';
@@ -910,6 +915,9 @@ function RetentionSweeper() {
       mx.getRooms().forEach((room) => {
         const maxLifetime = getRoomRetentionMs(room);
         if (!maxLifetime) return;
+        // [Gitea #298] Never delete under a policy that is less than a day old.
+        const setAt = getRoomRetentionSetAt(room);
+        if (setAt === undefined || !retentionDeletesAllowed(setAt, now)) return;
         room
           .getLiveTimeline()
           .getEvents()