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/ThreadPanel.css.ts

@@ -0,0 +1,30 @@
+import { style } from '@vanilla-extract/css';
+import { config, toRem } from 'folds';
+
+export const ThreadPanel = style({
+  width: toRem(360),
+  '@media': {
+    '(max-width: 750px)': {
+      position: 'fixed',
+      inset: 0,
+      width: '100%',
+      zIndex: 500,
+    },
+  },
+});
+
+export const ThreadPanelHeader = style({
+  flexShrink: 0,
+  padding: `0 ${config.space.S200} 0 ${config.space.S300}`,
+  borderBottomWidth: config.borderWidth.B300,
+});
+
+export const ThreadPanelContent = style({
+  position: 'relative',
+  overflow: 'hidden',
+});
+
+export const ThreadPanelInput = style({
+  padding: config.space.S200,
+  borderTopWidth: config.borderWidth.B300,
+});