fix: reset scroll ref on conversation change

prevLengthRef.current was never zeroed when conversationId changed,
so the first batch of messages in a new (shorter) conversation would
not trigger scrollIntoView. Reset it in the polling useEffect cleanup.

src/components/MessageList.tsx

@@ -32,7 +32,7 @@ export default function MessageList({ conversationId, refreshKey }: Props) {
     load();
     // ponytail: polling every 3s — replace with WebSocket when server supports it
     const id = setInterval(load, 3000);
-    return () => { cancelled = true; clearInterval(id); };
+    return () => { cancelled = true; clearInterval(id); prevLengthRef.current = 0; };
   // Fix 2: refreshKey in deps triggers immediate reload after send
   }, [conversationId, refreshKey]);