feat(threads): Thread Panel — full side drawer (P3-8)

Right-side thread drawer (MembersDrawer pattern; mobile fullscreen):
- ThreadPanel: header + close/Escape, ThreadTimeline, its own RoomInput
  (threadRootId prop; drafts/replies/uploads isolated per roomId::threadId;
  schedule + slash-commands off in threads v1) and threaded mark-as-read.
- ThreadTimeline: lean reimplementation over thread.liveTimeline — copied
  useTimelinePagination pattern (/relations back-pagination + decryption),
  virtualized, root event emphasized + "N replies" divider, reactions/edits/
  redactions, and a pending strip (chronological local echo never enters the
  thread timelineSet — rendered from LocalEchoUpdated instead).
- ThreadSummary chips on root messages (server-aggregated bundle or live
  Thread; unread badge via getThreadUnreadNotificationCount) keep threads
  discoverable now that replies leave the main timeline.
- Reply-in-Thread menu + thread indicators open the panel; deep links to
  thread events redirect into it.
- State: roomIdToActiveThreadIdAtomFamily + getThreadDraftKey (+18 tests).

Gates: tsc clean, eslint 0 errors, build OK, 616/617 tests (1 IDB skip).
Awaiting live QA; release note: threaded replies no longer render inline.

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

src/app/state/room/thread.ts

@@ -0,0 +1,22 @@
+import { atom, PrimitiveAtom } from 'jotai';
+import { atomFamily } from 'jotai/utils';
+
+const createActiveThreadIdAtom = () => atom<string | null>(null);
+export type TActiveThreadIdAtom = PrimitiveAtom<string | null>;
+
+/**
+ * Per-room "which thread is open in the panel" state. Mirrors
+ * `roomIdToReplyDraftAtomFamily` in `roomInputDrafts.ts` — the same atom
+ * instance is returned for the same roomId, so a room's panel state survives
+ * remounts.
+ */
+export const roomIdToActiveThreadIdAtomFamily = atomFamily<string, TActiveThreadIdAtom>(() =>
+  createActiveThreadIdAtom(),
+);
+
+/**
+ * Key used to scope a thread's composer drafts (message/reply/upload) away from
+ * the main room composer, e.g. `"!room:server::$rootEventId"`.
+ */
+export const getThreadDraftKey = (roomId: string, threadRootId: string): string =>
+  `${roomId}::${threadRootId}`;