fix(ui): P0 线程切换时调用 switchThread 彻底清除 error 状态

useStream 的 error 跨线程持久化，切换 threadId 时不自动重置。
useEffect 监听 currentThreadId 变化，调用 thread.switchThread()
清除 error、messages 等所有旧状态，从根本上解决切换线程后
红色错误横幅残留问题。

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

langgraph/src/main.tsx

@@ -95,6 +95,11 @@ function App() {
     return () => window.removeEventListener("open-canvas", handler);
   }, []);
 
+  // Clear error and stale stream state when thread switches
+  useEffect(() => {
+    thread.switchThread(currentThreadId);
+  }, [currentThreadId]); // eslint-disable-line react-hooks/exhaustive-deps
+
   // Sync concurrent status with thread loading state
   useEffect(() => {
     if (!thread.isLoading && concurrentStatus !== "cancelling") {