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/features/room/thread/threadSummary.ts

@@ -0,0 +1,55 @@
+import { EventStatus, IThreadBundledRelationship, MatrixEvent, RelationType } from 'matrix-js-sdk';
+
+export type ThreadSummaryData = {
+  count: number;
+  latestTs: number | undefined;
+};
+
+/**
+ * Summary data for a thread root's "N replies" chip.
+ *
+ * Prefers the live {@link Thread} object when it exists (it reflects local
+ * echo + pagination), otherwise falls back to the server-aggregated bundle
+ * (`unsigned['m.relations']['m.thread']`) so the chip renders before any
+ * Thread object has been created. Returns `undefined` when the root has no
+ * thread at all.
+ */
+export const getThreadSummary = (rootEvent: MatrixEvent): ThreadSummaryData | undefined => {
+  const thread = rootEvent.getThread();
+  if (thread) {
+    const lastReply = thread.lastReply();
+    return {
+      count: thread.length,
+      latestTs: lastReply?.getTs(),
+    };
+  }
+
+  const bundle = rootEvent.getServerAggregatedRelation<IThreadBundledRelationship>(
+    RelationType.Thread,
+  );
+  if (bundle) {
+    return {
+      count: bundle.count,
+      latestTs: bundle.latest_event?.origin_server_ts,
+    };
+  }
+
+  return undefined;
+};
+
+/**
+ * True when `event` is a still-in-flight (local echo) reply belonging to the
+ * given thread root. Used to render the pending strip, since pending thread
+ * sends never enter the thread's timelineSet.
+ */
+export const isPendingThreadReply = (event: MatrixEvent, threadRootId: string): boolean => {
+  const { status } = event;
+  if (status !== EventStatus.SENDING && status !== EventStatus.NOT_SENT) return false;
+
+  // Prefer the SDK's resolved thread root id; fall back to the raw relation
+  // content for events the SDK hasn't associated with a thread yet.
+  if (event.threadRootId === threadRootId) return true;
+
+  const relation = event.getRelation();
+  return relation?.rel_type === RelationType.Thread && relation.event_id === threadRootId;
+};