fix(desktop): remove chat-switch reveal flicker (decouple from reactions)

The residual flicker on chat switch was a loading/reveal artifact, not scroll.
listReady gated the MessageList reveal on reactionsReady OR a 300ms timeout, so
on a cache-hit switch (messages already present from the first render) the list
sat at opacity:0 for up to 300ms and then popped in. Drop the reactions/timeout
gate: reveal as soon as messages exist. Reaction chips stream in a beat later;
because the list is pinned to the bottom their height growth re-pins with no
visible jump, and MessageList still defers its own reveal a few frames until the
row-height measurement settles so it appears already at the final bottom.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/desktop/src/pages/ConversationPage.tsx

@@ -144,21 +144,18 @@ export function ConversationPage() {
     byMessage: reactionsByMessage,
     toggle: toggleReaction,
     voteExclusive: votePoll,
-    ready: reactionsReady,
   } = useMessageReactions(messageIds, session?.user.id);
 
-  // Deferred-reveal gate for MessageList: keep the list hidden until messages
-  // AND their reactions (the main post-paint height changer) are loaded, so the
-  // chat opens already-stable instead of flickering through the load cascade.
-  // A 300 ms max-timeout ensures a slow/empty reactions fetch never hangs it.
-  const [revealTimedOut, setRevealTimedOut] = useState(false);
-  useEffect(() => {
-    setRevealTimedOut(false);
-    if (!id || loading || messages.length === 0) return;
-    const tmo = window.setTimeout(() => setRevealTimedOut(true), 300);
-    return () => window.clearTimeout(tmo);
-  }, [id, loading, messages.length]);
-  const listReady = !loading && messages.length > 0 && (reactionsReady || revealTimedOut);
+  // Reveal gate for MessageList: as soon as messages exist (cache hit = first
+  // render, so no spinner and no wait), let the list reveal. We deliberately do
+  // NOT gate on reactions readiness: on a cache-hit chat switch the messages are
+  // already present, and gating on the async reactions fetch held the list at
+  // opacity:0 for up to 300ms and then "popped" it in — that was the residual
+  // chat-switch flicker. Reaction chips stream in a beat later; because the list
+  // is pinned to the bottom, their height growth re-pins with no visible jump.
+  // MessageList still defers its own reveal a few frames until the row-height
+  // measurement settles, so the list still appears already at the final bottom.
+  const listReady = !loading && messages.length > 0;
 
   const myId = session?.user.id;