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/useThread.ts

@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useState } from 'react';
 import {
+  EventStatus,
   EventTimeline,
   MatrixClient,
   MatrixEvent,
@@ -121,7 +122,15 @@ export const useThreadPendingEvents = (
 
         const alreadyInThread =
           eventId !== undefined && thread?.findEventById(eventId) !== undefined;
-        const stillPending = isPendingThreadReply(event, threadRootId) && !alreadyInThread;
+        // Keep a tracked event through the SENT window too: the /send response
+        // flips status to SENT before /sync delivers the event into the thread
+        // timeline — dropping it there would make the message flash out of view.
+        // It falls out on the next LocalEchoUpdated once findEventById sees it.
+        const trackedAndAwaitingSync =
+          event.status === EventStatus.SENT &&
+          prev.some((e) => e === event || (eventId !== undefined && e.getId() === eventId));
+        const stillPending =
+          !alreadyInThread && (isPendingThreadReply(event, threadRootId) || trackedAndAwaitingSync);
 
         if (stillPending) return [...without, event];
         return without.length === prev.length ? prev : without;