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/hooks/useThreadSummary.ts

@@ -0,0 +1,57 @@
+import { useEffect, useState } from 'react';
+import {
+  MatrixEvent,
+  NotificationCountType,
+  Room,
+  RoomEvent,
+  RoomEventHandlerMap,
+  ThreadEvent,
+} from 'matrix-js-sdk';
+import { getThreadSummary, ThreadSummaryData } from '../features/room/thread/threadSummary';
+
+/**
+ * Reactive thread summary + unread count for a root event's "N replies" chip.
+ *
+ * Re-computes the summary on `ThreadEvent.Update` (the SDK re-emits this on the
+ * root MatrixEvent) and the unread count on `RoomEvent.UnreadNotifications`.
+ */
+export const useThreadSummary = (
+  rootEvent: MatrixEvent,
+  room: Room,
+): { summary: ThreadSummaryData | undefined; unread: number } => {
+  const threadId = rootEvent.getId();
+
+  const [summary, setSummary] = useState<ThreadSummaryData | undefined>(() =>
+    getThreadSummary(rootEvent),
+  );
+  const [unread, setUnread] = useState<number>(() =>
+    threadId
+      ? (room.getThreadUnreadNotificationCount(threadId, NotificationCountType.Total) ?? 0)
+      : 0,
+  );
+
+  useEffect(() => {
+    const refreshSummary = () => setSummary(getThreadSummary(rootEvent));
+    const refreshUnread = () => {
+      if (!threadId) return;
+      setUnread(room.getThreadUnreadNotificationCount(threadId, NotificationCountType.Total) ?? 0);
+    };
+
+    refreshSummary();
+    refreshUnread();
+
+    const handleUnread: RoomEventHandlerMap[RoomEvent.UnreadNotifications] = (_counts, tId) => {
+      if (tId && tId !== threadId) return;
+      refreshUnread();
+    };
+
+    rootEvent.on(ThreadEvent.Update, refreshSummary);
+    room.on(RoomEvent.UnreadNotifications, handleUnread);
+    return () => {
+      rootEvent.removeListener(ThreadEvent.Update, refreshSummary);
+      room.removeListener(RoomEvent.UnreadNotifications, handleUnread);
+    };
+  }, [rootEvent, room, threadId]);
+
+  return { summary, unread };
+};