fix(P1): load thread state before switching threadId to prevent welcome flash

Previously: setHistoricalMessages([]) + setCurrentThreadId() fired together,
causing a render where both thread.messages (cleared by switchThread) and
historicalMessages were empty → welcome screen shown.

Fix: await loadThreadState() first, then set historicalMessages/ui and
currentThreadId atomically. historicalMessages is populated before the
first render after the switch, so the welcome screen never flashes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

langgraph/src/hooks/useConversation.ts

@@ -43,13 +43,13 @@ export function useConversation(
     } catch {
       setInput("");
     }
-    setHistoricalMessages([]);
-    setHistoricalUi([]);
+    // Load state BEFORE switching threadId so historicalMessages is populated
+    // when the first render after the switch fires — avoids flash of welcome screen.
+    const state = await loadThreadState(client, threadId);
+    setHistoricalMessages(state.messages.length ? state.messages : []);
+    setHistoricalUi(state.ui.length ? (state.ui as UIMsgLocal[]) : []);
     setCurrentThreadId(threadId);
     setSidebarOpen(false);
-    const state = await loadThreadState(client, threadId);
-    if (state.messages.length) setHistoricalMessages(state.messages);
-    if (state.ui.length) setHistoricalUi(state.ui as UIMsgLocal[]);
   }, [currentThreadId, input, client, setInput]);
 
   const handleDeleteThread = useCallback(async (threadId: string) => {