fix: P1 体验修复 — 隐藏空对话导出按钮 + 发送后自动滚动

- ConfigPanel: activeMessages 为空时隐藏导出按钮，减少界面噪音
- main.tsx: doSubmit 后 scrollIntoView 确保用户消息可见

Confidence: high
Scope-risk: narrow

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

langgraph/src/components/ConfigPanel.tsx

@@ -42,8 +42,10 @@ export function ConfigPanel({
   return (
     <div className="shrink-0 border-t border-border px-4 pt-2 pb-0 max-w-3xl mx-auto w-full">
       <div ref={configPanelRef} className="relative flex items-center gap-2">
-        {/* Export button */}
-        <ExportButton messages={activeMessages as Array<Record<string, unknown>>} />
+        {/* Export button — hidden when no messages */}
+        {activeMessages.length > 0 && (
+          <ExportButton messages={activeMessages as Array<Record<string, unknown>>} />
+        )}
         {/* Summary bar */}
         <button
           type="button"

langgraph/src/main.tsx

@@ -251,6 +251,9 @@ function App() {
       },
     );
 
+    // Scroll to bottom after submitting so user message stays visible
+    setTimeout(() => bottomRef.current?.scrollIntoView({ behavior: "smooth" }), 100);
+
     if (currentThreadId && text) {
       const existingTitle = threads.find((t) => t.thread_id === currentThreadId)?.metadata?.title as string | undefined;
       applyThreadTitle(currentThreadId, text, activeMessages.length, existingTitle);