fix(ui): P1-1 彻底修复线程切换竞态 — 抑制 useStream 旧数据

useStream 在 threadId 变更后仍有一个渲染周期保留旧线程的 messages。
新增 threadJustSwitched 标记，切换时忽略 thread.messages，强制走
historicalMessages（新线程为空），保证欢迎页必然显示。

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

langgraph/src/main.tsx

@@ -155,16 +155,20 @@ function App() {
   // ── Active messages ──────────────────────────────────────────────────────
   const lastMessagesRef = useRef<Message[]>([]);
   const lastThreadIdRef = useRef<string | null>(null);
-  // Clear stale messages when thread switches to avoid showing previous thread's content
-  if (currentThreadId !== lastThreadIdRef.current) {
+  // Detect thread switch — when threadId changes, thread.messages still holds stale data
+  // for the current render cycle (useStream is async). Track this to suppress stale data.
+  const threadJustSwitched = currentThreadId !== lastThreadIdRef.current;
+  if (threadJustSwitched) {
     lastMessagesRef.current = [];
     lastThreadIdRef.current = currentThreadId;
   }
-  if (thread.messages.length > 0) {
+  // Only trust thread.messages when useStream has caught up to the current threadId
+  const threadMessagesValid = !threadJustSwitched && thread.messages.length > 0;
+  if (threadMessagesValid) {
     lastMessagesRef.current = thread.messages;
   }
   const activeMessages: Message[] = deduplicateMessages(
-    thread.messages.length > 0
+    threadMessagesValid
       ? thread.messages
       : lastMessagesRef.current.length > 0
         ? lastMessagesRef.current