Address findings from 2 review agents on the bookmark sort/group feature:
- Flash on open: the persisted-sort atom now uses getOnInit so the saved
sort applies on the first render instead of briefly showing Newest and
reordering after mount.
- Stale collapse state: prune collapsed roomIds that no longer have any
bookmark, so a room re-saved later doesn't reappear pre-collapsed and
the Set can't grow unbounded across a session.
- Corrupt persisted value: validate the stored sort with a new
isBookmarkSort type guard, normalizing anything unexpected to Newest so
exactly one sort button is always active.
- a11y: room group headers now expose an explicit aria-label
("<room>, N saved messages") instead of announcing the avatar alt and
the visible name twice with a bare count, plus aria-controls linking the
header to its collapsible content region.
- Layout: move the sort control to its own toolbar row so the three
buttons don't crowd the count text in the narrow (266px) panel.
- Memoize filtered/sortedItems/groups for consistency with renderItem.
Adds unit tests for isBookmarkSort, group-order tie-break, and
groupBookmarksByRoom immutability.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>