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/Room.tsx

@@ -22,6 +22,8 @@ import { callChatAtom } from '../../state/callEmbed';
 import { CallChatView } from './CallChatView';
 import { useCallEmbed } from '../../hooks/useCallEmbed';
 import { useCallMembers, useCallSession } from '../../hooks/useCall';
+import { roomIdToActiveThreadIdAtomFamily } from '../../state/room/thread';
+import { ThreadPanel } from './thread';
 
 export function Room() {
   const { eventId } = useParams();
@@ -33,6 +35,8 @@ export function Room() {
   const callEmbed = useCallEmbed();
 
   const [isDrawer] = useSetting(settingsAtom, 'isPeopleDrawer');
+  const activeThreadId = useAtomValue(roomIdToActiveThreadIdAtomFamily(room.roomId));
+  const setActiveThreadId = useSetAtom(roomIdToActiveThreadIdAtomFamily(room.roomId));
   const galleryOpen = useAtomValue(mediaGalleryAtom);
   const setGalleryOpen = useSetAtom(mediaGalleryAtom);
   const [hideActivity] = useSetting(settingsAtom, 'hideActivity');
@@ -90,6 +94,19 @@ export function Room() {
             <MediaGallery key={room.roomId} room={room} onClose={() => setGalleryOpen(false)} />
           </>
         )}
+        {!callView && activeThreadId && (
+          <>
+            {screenSize === ScreenSize.Desktop && (
+              <Line variant="Background" direction="Vertical" size="300" />
+            )}
+            <ThreadPanel
+              key={`${room.roomId}${activeThreadId}`}
+              room={room}
+              threadId={activeThreadId}
+              requestClose={() => setActiveThreadId(null)}
+            />
+          </>
+        )}
         {!callView && isDrawer && (
           <>
             {screenSize === ScreenSize.Desktop && (