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.test.ts

@@ -0,0 +1,104 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { sortBookmarks, groupBookmarksByRoom } from './bookmarks';
+import { Bookmark } from '../hooks/useBookmarks';
+
+const bk = (eventId: string, roomId: string, savedAt: number, roomName = roomId): Bookmark => ({
+  eventId,
+  roomId,
+  savedAt,
+  roomName,
+  previewText: eventId,
+});
+
+test('sortBookmarks newest orders by savedAt descending', () => {
+  const input = [bk('a', 'r1', 100), bk('b', 'r1', 300), bk('c', 'r1', 200)];
+  const out = sortBookmarks(input, 'newest');
+  assert.deepEqual(
+    out.map((b) => b.eventId),
+    ['b', 'c', 'a'],
+  );
+});
+
+test('sortBookmarks oldest orders by savedAt ascending', () => {
+  const input = [bk('a', 'r1', 100), bk('b', 'r1', 300), bk('c', 'r1', 200)];
+  const out = sortBookmarks(input, 'oldest');
+  assert.deepEqual(
+    out.map((b) => b.eventId),
+    ['a', 'c', 'b'],
+  );
+});
+
+test('sortBookmarks room falls back to newest for a flat list', () => {
+  const input = [bk('a', 'r1', 100), bk('b', 'r2', 300)];
+  const out = sortBookmarks(input, 'room');
+  assert.deepEqual(
+    out.map((b) => b.eventId),
+    ['b', 'a'],
+  );
+});
+
+test('sortBookmarks breaks savedAt ties deterministically by eventId', () => {
+  const input = [bk('z', 'r1', 100), bk('a', 'r1', 100), bk('m', 'r1', 100)];
+  const newest = sortBookmarks(input, 'newest');
+  const oldest = sortBookmarks(input, 'oldest');
+  // Equal timestamps → ascending eventId in both directions.
+  assert.deepEqual(
+    newest.map((b) => b.eventId),
+    ['a', 'm', 'z'],
+  );
+  assert.deepEqual(
+    oldest.map((b) => b.eventId),
+    ['a', 'm', 'z'],
+  );
+});
+
+test('sortBookmarks does not mutate its input', () => {
+  const input = [bk('a', 'r1', 100), bk('b', 'r1', 300)];
+  const before = input.map((b) => b.eventId);
+  sortBookmarks(input, 'oldest');
+  assert.deepEqual(
+    input.map((b) => b.eventId),
+    before,
+  );
+});
+
+test('groupBookmarksByRoom buckets by room, newest-first within a group', () => {
+  const input = [
+    bk('a', 'r1', 100),
+    bk('b', 'r2', 500),
+    bk('c', 'r1', 300),
+    bk('d', 'r2', 200),
+  ];
+  const groups = groupBookmarksByRoom(input);
+  assert.equal(groups.length, 2);
+  const r1 = groups.find((g) => g.roomId === 'r1')!;
+  const r2 = groups.find((g) => g.roomId === 'r2')!;
+  assert.deepEqual(
+    r1.items.map((b) => b.eventId),
+    ['c', 'a'],
+  );
+  assert.deepEqual(
+    r2.items.map((b) => b.eventId),
+    ['b', 'd'],
+  );
+});
+
+test('groupBookmarksByRoom orders groups by most-recent save (active rooms first)', () => {
+  const input = [bk('a', 'r1', 100), bk('c', 'r1', 300), bk('b', 'r2', 500)];
+  // r2's newest is 500, r1's newest is 300 → r2 first.
+  const groups = groupBookmarksByRoom(input);
+  assert.deepEqual(
+    groups.map((g) => g.roomId),
+    ['r2', 'r1'],
+  );
+});
+
+test('groupBookmarksByRoom carries the stored room name', () => {
+  const groups = groupBookmarksByRoom([bk('a', 'r1', 100, 'General')]);
+  assert.equal(groups[0].roomName, 'General');
+});
+
+test('groupBookmarksByRoom returns empty for empty input', () => {
+  assert.deepEqual(groupBookmarksByRoom([]), []);
+});