fix(threads): review-wave fixes — decryption re-render, receipt dedupe, chip perf

Two-reviewer audit of the thread stack; confirmed findings fixed:
- ThreadTimeline: wrap encrypted events in EncryptedContent so a live-arriving
  E2EE reply re-renders when its key decrypts (decryption emits neither
  RoomEvent.Timeline nor ThreadEvent.Update — previously stuck at "Unable to
  decrypt").
- ThreadPanel: mark-read deduped on the latest event id (RoomEvent.Timeline
  re-emits per backfilled event/edit/reaction; previously up to N receipt POSTs
  per panel open) + rejection handled with retry.
- RoomTimeline: ThreadSummary chips now mount only for events carrying thread
  data (each chip holds a room-level listener; one per rendered message would
  blow the SDK's 100-listener emitter cap) with a single room-level
  ThreadEvent.New tick for new-thread liveness.
- useThreadPendingEvents: keep a sent reply visible through the /send-response→
  /sync window (was flashing out of the pending strip before landing).
- ThreadTimeline: reseed the window on RoomEvent.TimelineReset (gappy sync left
  a detached timeline).

Documented-acceptable (reviewer-noted): thread typing shows as room typing (no
per-thread typing in the spec; Element matches), thread panel + members drawer
can be open together, scheduled-send is thread-unaware but unreachable there.

Gates: tsc clean, eslint 0 errors, build OK, 616/617 tests (1 IDB skip).

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

src/app/features/room/thread/ThreadPanel.tsx

@@ -95,10 +95,32 @@ export function ThreadPanel({ room, threadId, requestClose }: ThreadPanelProps)
   );
 
   // Mark the thread read when the panel is open and on each new thread event.
+  // Deduped on the latest event id: RoomEvent.Timeline re-emits per event during
+  // backfill and for every edit/reaction, and sendReadReceipt POSTs
+  // unconditionally — without the guard, opening a thread with N replies would
+  // fire up to N receipt requests at the same event.
+  const lastReadEventIdRef = useRef<string | undefined>(undefined);
   useEffect(() => {
+    lastReadEventIdRef.current = undefined;
     if (!thread) return undefined;
     const markRead = () => {
-      markThreadAsRead(mx, thread, privateReadReceipts);
+      const events = thread.liveTimeline.getEvents();
+      let latestId: string | undefined;
+      for (let i = events.length - 1; i >= 0; i -= 1) {
+        const evt = events[i];
+        if (evt && !evt.isSending()) {
+          latestId = evt.getId() ?? undefined;
+          break;
+        }
+      }
+      if (!latestId || latestId === lastReadEventIdRef.current) return;
+      lastReadEventIdRef.current = latestId;
+      markThreadAsRead(mx, thread, privateReadReceipts).catch(() => {
+        // Allow a retry on the next event if the receipt POST failed.
+        if (lastReadEventIdRef.current === latestId) {
+          lastReadEventIdRef.current = undefined;
+        }
+      });
     };
     markRead();
     thread.on(ThreadEvent.NewReply, markRead);