feat(bookmarks): sort & group saved messages by room

The Saved Messages panel showed bookmarks in one fixed order (newest
save first) with no way to reorganize. Add a Newest / Oldest / By-room
segmented sort control to the panel toolbar. In "By room" mode the list
renders collapsible per-room sections, with groups ordered by their most
recently saved message so active rooms float to the top. The chosen sort
persists across panel opens via a localStorage-backed atom.

Ordering and grouping are pure functions in utils/bookmarks.ts
(sortBookmarks, groupBookmarksByRoom) with deterministic eventId
tie-breaks, covered by bookmarks.test.ts (9 tests). No change to the
bookmark data model, account-data schema, useBookmarks, or how bookmarks
are created; search still feeds the sorter/grouper unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/utils/bookmarks.ts

@@ -0,0 +1,59 @@
+import { Bookmark } from '../hooks/useBookmarks';
+
+export type BookmarkSort = 'newest' | 'oldest' | 'room';
+
+export type BookmarkRoomGroup = {
+  roomId: string;
+  roomName: string;
+  items: Bookmark[];
+};
+
+// Deterministic newest-first comparator: most recently saved first, ties broken
+// by eventId so the order is stable regardless of input order (keeps tests and
+// re-renders from shuffling equal-timestamp entries).
+const byNewest = (a: Bookmark, b: Bookmark): number =>
+  b.savedAt - a.savedAt || (a.eventId < b.eventId ? -1 : a.eventId > b.eventId ? 1 : 0);
+
+/**
+ * Return a new array of bookmarks ordered per the selected sort. `room` has no
+ * flat ordering of its own (grouping is a render concern) and falls back to
+ * newest-first so callers can still render a sensible flat list if they want.
+ */
+export function sortBookmarks(bookmarks: Bookmark[], sort: BookmarkSort): Bookmark[] {
+  const copy = [...bookmarks];
+  if (sort === 'oldest') {
+    // Oldest-first is the reverse ordering; keep the same eventId tie-break shape.
+    return copy.sort(
+      (a, b) => a.savedAt - b.savedAt || (a.eventId < b.eventId ? -1 : a.eventId > b.eventId ? 1 : 0),
+    );
+  }
+  return copy.sort(byNewest);
+}
+
+/**
+ * Bucket bookmarks by room. Items within a group are newest-first; groups are
+ * ordered by each group's most-recently-saved bookmark (active rooms float up),
+ * with the room's stored name (the panel overrides with the live name when the
+ * room is joined). Deterministic tie-breaks keep the output stable.
+ */
+export function groupBookmarksByRoom(bookmarks: Bookmark[]): BookmarkRoomGroup[] {
+  const groups = new Map<string, BookmarkRoomGroup>();
+  bookmarks.forEach((bk) => {
+    const existing = groups.get(bk.roomId);
+    if (existing) {
+      existing.items.push(bk);
+    } else {
+      groups.set(bk.roomId, { roomId: bk.roomId, roomName: bk.roomName, items: [bk] });
+    }
+  });
+
+  const result = [...groups.values()];
+  result.forEach((group) => group.items.sort(byNewest));
+  // Order groups by their newest item's savedAt (desc); tie-break by roomId.
+  result.sort((a, b) => {
+    const diff = b.items[0].savedAt - a.items[0].savedAt;
+    if (diff !== 0) return diff;
+    return a.roomId < b.roomId ? -1 : a.roomId > b.roomId ? 1 : 0;
+  });
+  return result;
+}