fix(threads): harden threads list after review

Address findings from 3 review agents on the Threads list panel:

- Last-activity accuracy (SDK): sort key and the "last reply <time>"
  label now use thread.replyToEvent.getTs() (server bundle latest_event)
  instead of lastReply(), which returns the ROOT time until each thread's
  replies lazily paginate (or permanently on fetch error). Applied to the
  hook signature too.

- Live-refresh completeness (correctness): the useRoomThreads signature
  now includes thread.length and the root event's replacingEventId, so a
  mid-thread redaction (reply count) and a root-message edit (row snippet)
  refresh the row live instead of going stale.

- a11y: the row's aria-label was the button's whole accessible name,
  hiding the snippet/count/unread from screen readers. It now describes
  the thread ("Open thread by <name>, unread, N replies, last reply ..").

- Unread badge: replaced the bare green dot (Success = the mention color)
  with the app-wide UnreadBadge, using the Highlight count so mentions
  render red and ordinary unread renders secondary, matching room-nav.

- Hover/focus affordance: the clickable row moved its inline styles to a
  css class with token-based :hover / :active backgrounds.

- Participant pile now also includes the last replier from the bundle.

- Stabilized the panel's onClose/onOpenThread with useCallback so its
  Escape listener isn't re-subscribed every Room render. Added
  filter->sort pipeline + all/participating immutability tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/hooks/useRoomThreads.ts

@@ -1,16 +1,23 @@
 import { useEffect, useRef, useState } from 'react';
 import { NotificationCountType, Room, RoomEvent, Thread, ThreadEvent } from 'matrix-js-sdk';
 
-// Cheap signature over the fields the Threads list actually depends on (id,
-// last-activity ts, unread count, participation). We only push a new snapshot
-// when this changes, so noisy repeated ThreadEvent.Update / UnreadNotifications
-// re-emits that don't alter the list don't cause a re-render.
+// Cheap signature over the fields the Threads list actually renders on (id,
+// last-activity ts, reply count, unread, participation, and a root-edit token).
+// We only push a new snapshot when this changes, so noisy repeated
+// ThreadEvent.Update / UnreadNotifications re-emits that don't alter the list
+// don't cause a re-render.
+// `replyToEvent` reads the server bundle's latest_event, so last-activity is
+// accurate immediately (before/without lazily paginating each thread's replies);
+// `length` covers reply-count changes (e.g. a mid-thread redaction) and
+// `replacingEventId` covers a root-message edit changing the row snippet.
 const signatureOf = (room: Room, threads: Thread[]): string =>
   threads
     .map((t) => {
-      const ts = t.lastReply()?.getTs() ?? t.rootEvent?.getTs() ?? 0;
+      const ts = t.replyToEvent?.getTs() ?? t.rootEvent?.getTs() ?? 0;
       const unread = room.getThreadUnreadNotificationCount(t.id, NotificationCountType.Total) ?? 0;
-      return `${t.id}:${ts}:${unread}:${t.hasCurrentUserParticipated ? 1 : 0}`;
+      const rootEdit = t.rootEvent?.replacingEventId() ?? '';
+      const participated = t.hasCurrentUserParticipated ? 1 : 0;
+      return `${t.id}:${ts}:${t.length}:${unread}:${participated}:${rootEdit}`;
     })
     .sort()
     .join('|');