Fix scroll-to-bottom button not reset on thread switch

Reset isAtEndRef and showScrollToBottom when activeThread changes so stale
scroll state from a previous thread does not persist into the new one.

Author: cursoragent

apps/web/src/components/ChatView.tsx

@@ -1968,6 +1968,8 @@ export default function ChatView(props: ChatViewProps) {
   useEffect(() => {
     setExpandedWorkGroups({});
     setPullRequestDialogState(null);
+    isAtEndRef.current = true;
+    setShowScrollToBottom(false);
     if (planSidebarOpenOnNextThreadRef.current) {
       planSidebarOpenOnNextThreadRef.current = false;
       setPlanSidebarOpen(true);