feat(threads): room-level Threads list panel

Lotus could view one thread at a time but had no overview of a room's
threads. Add a Threads list side panel, opened from a new Threads toggle
in the room header (mirrors the gallery/widgets toggles).

- Lists every thread with a rich row: root sender + snippet, unread dot,
  "N replies - last reply <time>", and a participant avatar pile.
- Segmented filter (All / Unread / Participating) and sort (Recent /
  Oldest by last-reply time), both persisted in localStorage
  (cinny_threads_filter_v1 / cinny_threads_sort_v1) and normalized via
  type guards.
- Clicking a row opens the existing single-thread ThreadPanel by reusing
  setActiveThreadId; reading it clears the row's unread badge live.
- Stays live via ThreadEvent.New/NewReply/Update/Delete +
  RoomEvent.UnreadNotifications, with a signature guard to avoid churn,
  and is virtualized (@tanstack/react-virtual) for busy rooms.

Reuses room.getThreads()/fetchRoomThreads(), thread.hasCurrentUser-
Participated / lastReply() / length, getThreadUnreadNotificationCount
(muted threads zeroed), useMemberAvatar/StackedAvatar/UserAvatar,
scaleSystemEmoji/trimReplyFromBody, UnreadBadge, and the Bookmarks-panel
segmented-control + localStorage-atom patterns. Filter/sort logic is pure
in utils/threadList.ts with 8 unit tests. New panel is wired into
Room.tsx's mutually-exclusive content-panel switching.

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

src/app/state/threadsList.ts

@@ -0,0 +1,4 @@
+import { atom } from 'jotai';
+
+// Whether the room-level Threads list panel is open (mirrors mediaGalleryAtom).
+export const threadsListAtom = atom<boolean>(false);