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/components/message/Reply.tsx

@@ -61,6 +61,7 @@ type ReplyProps = {
   replyEventId: string;
   threadRootId?: string | undefined;
   onClick?: MouseEventHandler | undefined;
+  onThreadClick?: ((threadRootId: string) => void) | undefined;
   getMemberPowerTag?: GetMemberPowerTag;
   accessibleTagColors?: Map<string, string>;
   legacyUsernameColor?: boolean;
@@ -74,6 +75,7 @@ export const Reply = as<'div', ReplyProps>(
       replyEventId,
       threadRootId,
       onClick,
+      onThreadClick,
       getMemberPowerTag,
       accessibleTagColors,
       legacyUsernameColor,
@@ -110,7 +112,7 @@ export const Reply = as<'div', ReplyProps>(
           <ThreadIndicator
             as="button"
             data-event-id={threadRootId}
-            onClick={onClick}
+            onClick={onThreadClick ? () => onThreadClick(threadRootId) : onClick}
             aria-label="View thread"
           />
         )}