fix(bookmarks): harden sort/group after review

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>

src/app/utils/bookmarks.ts

@@ -2,6 +2,13 @@ import { Bookmark } from '../hooks/useBookmarks';
 
 export type BookmarkSort = 'newest' | 'oldest' | 'room';
 
+const BOOKMARK_SORTS: readonly BookmarkSort[] = ['newest', 'oldest', 'room'];
+
+/** Type guard for persisted/untrusted sort values (localStorage can hold stale/corrupt data). */
+export function isBookmarkSort(value: unknown): value is BookmarkSort {
+  return typeof value === 'string' && (BOOKMARK_SORTS as readonly string[]).includes(value);
+}
+
 export type BookmarkRoomGroup = {
   roomId: string;
   roomName: string;