fix(P1): restore historical thread message loading after thread switch

The threadJustSwitched suppression was too aggressive — it cleared
lastMessagesRef on every thread switch, then relied on historicalMessages
which loads async and may still be empty. Historical thread messages
would never appear.

Simplified fix: thread.switchThread() already clears thread.messages
when switching, so thread.messages is always safe to trust directly.
Use thread.messages when non-empty, else fall back to historicalMessages.
Removes threadJustSwitched / lastMessagesRef / lastThreadIdRef entirely.

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

langgraph/src/main.tsx

@@ -153,26 +153,12 @@ function App() {
   useEffect(() => { autoResizeTextarea(textareaRef); }, [input]);
 
   // ── Active messages ──────────────────────────────────────────────────────
-  const lastMessagesRef = useRef<Message[]>([]);
-  const lastThreadIdRef = useRef<string | null>(null);
-  // 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;
-  }
-  // 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;
-  }
+  // thread.switchThread() is called in useEffect whenever currentThreadId changes,
+  // which clears thread.messages. So thread.messages is always safe to trust:
+  // - empty = thread just switched or genuinely empty → fall back to historicalMessages
+  // - non-empty = useStream has loaded messages for the current thread
   const activeMessages: Message[] = deduplicateMessages(
-    threadMessagesValid
-      ? thread.messages
-      : lastMessagesRef.current.length > 0
-        ? lastMessagesRef.current
-        : historicalMessages
+    thread.messages.length > 0 ? thread.messages : historicalMessages
   );
 
   // Completed / failed tool IDs