fix(P0): 修复新线程发消息后跳回欢迎屏

根因：ensureThread() 通过 setCurrentThreadId 设置新线程ID，但 React 状态
未同步刷新，useStream 仍绑定 threadId=undefined，导致 submit() 提交到
错误的线程。

修复：在 submit 前手动调 thread.switchThread(activeThreadId)，确保
useStream 立即指向新创建的线程。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

langgraph/src/main.tsx

@@ -217,11 +217,12 @@ function App() {
       setConcurrentStatus("idle");
       return;
     }
-    // For a newly created thread, currentThreadId just changed so the useEffect
-    // would call thread.switchThread() which clears thread.messages mid-stream.
-    // Skip it — useStream already uses the updated threadId prop from currentThreadId.
+    // For a newly created thread, currentThreadId just changed via setCurrentThreadId
+    // but React state hasn't flushed yet — useStream still has threadId=undefined.
+    // Manually switch useStream to the new thread so submit() targets the right thread.
     if (!currentThreadId) {
       skipNextSwitchRef.current = true;
+      thread.switchThread(activeThreadId);
     }
 
     try { localStorage.removeItem(`draft_${activeThreadId}`); } catch { /* ignore */ }