fix(P0): prevent thread.switchThread from clearing messages mid-stream

When ensureThread() creates a new thread and calls setCurrentThreadId,
the useEffect fires thread.switchThread() again after doSubmit already
called it — wiping thread.messages mid-stream and showing the welcome
screen instead of AI replies.

Fix: add skipNextSwitchRef. When doSubmit creates a new thread and
manually calls thread.switchThread(), it sets the flag so the
subsequent useEffect is a no-op. Only needed for new threads (when
currentThreadId was null); existing-thread submits don't change
currentThreadId so the useEffect never fires.

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

langgraph/src/main.tsx

@@ -63,6 +63,8 @@ function App() {
   type ConcurrentStatus = "idle" | "generating" | "stopping" | "cancelling";
   const [concurrentStatus, setConcurrentStatus] = useState<ConcurrentStatus>("idle");
   const wasStoppedRef = useRef(false);
+  // Skip the switchThread useEffect when doSubmit already called it manually
+  const skipNextSwitchRef = useRef(false);
 
   interface SubmitPayload {
     text: string;
@@ -95,8 +97,13 @@ function App() {
     return () => window.removeEventListener("open-canvas", handler);
   }, []);
 
-  // Clear error and stale stream state when thread switches
+  // Clear error and stale stream state when thread switches.
+  // Skip when doSubmit already called switchThread manually (to avoid clearing mid-stream).
   useEffect(() => {
+    if (skipNextSwitchRef.current) {
+      skipNextSwitchRef.current = false;
+      return;
+    }
     thread.switchThread(currentThreadId);
   }, [currentThreadId]); // eslint-disable-line react-hooks/exhaustive-deps
 
@@ -210,7 +217,13 @@ function App() {
       setConcurrentStatus("idle");
       return;
     }
-    thread.switchThread(activeThreadId);
+    // Only need to switchThread manually when a new thread was just created.
+    // For existing threads, currentThreadId didn't change so useEffect won't fire.
+    // Setting the skip flag prevents the useEffect from re-firing and clearing messages.
+    if (!currentThreadId) {
+      skipNextSwitchRef.current = true;
+      thread.switchThread(activeThreadId);
+    }
 
     try { localStorage.removeItem(`draft_${activeThreadId}`); } catch { /* ignore */ }
     updateThreadLastActive(activeThreadId);