feat: add responsive react workbench

web/src/app/PlaceholderPage.tsx

@@ -0,0 +1,15 @@
+import { BookOpenText, MessageSquareText } from 'lucide-react'
+import { useLocation } from 'react-router-dom'
+
+export function PlaceholderPage() {
+  const documents = useLocation().pathname.startsWith('/documents')
+  const Icon = documents ? BookOpenText : MessageSquareText
+  return (
+    <section className="placeholder-page">
+      <div className="placeholder-page__mark"><Icon size={28} /></div>
+      <p className="eyebrow">KNOWLEDGE DESK</p>
+      <h1>{documents ? '文档工作区' : '知识库对话'}</h1>
+      <p>{documents ? '上传、索引与审阅你的资料。' : '基于可追溯证据展开研究。'}</p>
+    </section>
+  )
+}