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/features/room/thread/ThreadsListPanel.css.ts

@@ -1,5 +1,5 @@
 import { style } from '@vanilla-extract/css';
-import { config, toRem } from 'folds';
+import { color, config, toRem } from 'folds';
 
 export const ThreadsListPanel = style({
   width: toRem(340),
@@ -28,3 +28,17 @@ export const ThreadsListToolbar = style({
 export const ThreadsListContent = style({
   padding: config.space.S200,
 });
+
+export const ThreadRow = style({
+  width: '100%',
+  textAlign: 'left',
+  cursor: 'pointer',
+  border: 'none',
+  borderRadius: config.radii.R300,
+  padding: config.space.S300,
+  background: color.SurfaceVariant.Container,
+  selectors: {
+    '&:hover': { background: color.SurfaceVariant.ContainerHover },
+    '&:active': { background: color.SurfaceVariant.ContainerActive },
+  },
+});