fix: card occlusion by input bar, thread title update for existing threads

- Add h-32 spacer before bottomRef to prevent last card from being hidden behind input bar
- Fix thread title not updating: check existing title instead of only first message

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

langgraph/src/main.tsx

@@ -567,7 +567,8 @@ function App() {
       messageContent = text;
     }
 
-    const isFirstMessage = activeMessages.length === 0;
+    const existingTitle = threads.find((t) => t.thread_id === currentThreadId)?.metadata?.title;
+    const isFirstMessage = activeMessages.length === 0 || !existingTitle;
     const retryTool = pendingRetryToolRef.current;
     pendingRetryToolRef.current = null;
 
@@ -1234,6 +1235,8 @@ function App() {
             </div>
           )}
 
+          {/* Spacer to prevent last card from being hidden behind input bar */}
+          <div className="h-32" />
           <div ref={bottomRef} />
 
           {/* Scroll-to-bottom floating button */}