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.tsx

@@ -0,0 +1,49 @@
+import React from 'react';
+import { Badge, Box, Chip, Icon, Icons, Text, config } from 'folds';
+import { MatrixEvent, Room } from 'matrix-js-sdk';
+import { useThreadSummary } from '../../../hooks/useThreadSummary';
+import { useSetting } from '../../../state/hooks/settings';
+import { settingsAtom } from '../../../state/settings';
+import { timeDayMonthYear, timeHourMinute, today } from '../../../utils/time';
+
+type ThreadSummaryProps = {
+  rootEvent: MatrixEvent;
+  room: Room;
+  onOpen: (threadId: string) => void;
+};
+export function ThreadSummary({ rootEvent, room, onOpen }: ThreadSummaryProps) {
+  const { summary, unread } = useThreadSummary(rootEvent, room);
+  const [hour24Clock] = useSetting(settingsAtom, 'hour24Clock');
+
+  if (!summary || summary.count === 0) return null;
+
+  const { count, latestTs } = summary;
+  const latestStr =
+    latestTs !== undefined
+      ? today(latestTs)
+        ? timeHourMinute(latestTs, hour24Clock)
+        : timeDayMonthYear(latestTs)
+      : undefined;
+
+  return (
+    <Box style={{ marginTop: config.space.S200 }}>
+      <Chip
+        variant="SurfaceVariant"
+        radii="300"
+        before={<Icon size="50" src={Icons.Thread} />}
+        after={
+          unread > 0 ? <Badge variant="Success" fill="Solid" radii="Pill" size="200" /> : undefined
+        }
+        onClick={() => {
+          const threadId = rootEvent.getId();
+          if (threadId) onOpen(threadId);
+        }}
+      >
+        <Text size="T200">
+          {count === 1 ? '1 reply' : `${count} replies`}
+          {latestStr ? ` · ${latestStr}` : ''}
+        </Text>
+      </Chip>
+    </Box>
+  );
+}