优化 P0 工作台与访客 Widget，对齐设计稿 UI

- 工作台改为设计稿三栏：320 会话列表 + 聊天区 + 320 客户信息
- 会话列表展示头像色条、最后消息预览、相对时间与未读角标
- 消息方向对齐设计（访客左、客服右）并补系统会话条
- 访客 Widget 还原欢迎语、快捷问题、输入状态动画与实色顶栏
- 会话列表 API 返回 last_message 供列表预览

web/src/pages/WidgetPreview.tsx

@@ -1,23 +1,30 @@
-import { useState } from 'react'
 import VisitorChat from '@/widgets/VisitorChat'
 
 const WidgetPreview = () => {
-  const [showWidget, setShowWidget] = useState(true)
-
   return (
-    <div className="min-h-screen bg-neutral-100 flex flex-col">
-      <div className="flex-1 flex items-center justify-center">
-        <div className="text-center">
-          <h1 className="text-2xl font-bold text-neutral-800 mb-2">示例网站页面</h1>
-          <p className="text-neutral-400 mb-4">Widget 在右下角，发送消息会写入数据库</p>
-          {!showWidget && (
-            <button onClick={() => setShowWidget(true)} className="text-sm text-blue-500 hover:text-blue-600 underline">
-              重新显示 Widget
-            </button>
-          )}
+    <div className="min-h-screen bg-neutral-100 relative overflow-hidden">
+      {/* 模拟宿主站点骨架，贴近设计稿模糊背景 */}
+      <div className="max-w-[960px] mx-auto px-6 py-10 opacity-40 select-none pointer-events-none">
+        <div className="h-10 w-3/5 bg-neutral-200 rounded mb-4" />
+        <div className="h-4 w-[90%] bg-neutral-200 rounded mb-3" />
+        <div className="h-4 w-3/4 bg-neutral-200 rounded mb-6" />
+        <div className="flex gap-4 mb-6">
+          <div className="w-[200px] h-[120px] bg-neutral-200 rounded-lg" />
+          <div className="w-[200px] h-[120px] bg-neutral-200 rounded-lg" />
+          <div className="w-[200px] h-[120px] bg-neutral-200 rounded-lg" />
+        </div>
+        <div className="h-4 w-[85%] bg-neutral-200 rounded mb-3" />
+        <div className="h-4 w-[70%] bg-neutral-200 rounded mb-3" />
+        <div className="h-4 w-4/5 bg-neutral-200 rounded mb-6" />
+        <div className="h-[200px] w-full bg-neutral-200 rounded-lg" />
+      </div>
+      <div className="absolute inset-0 flex items-start justify-center pt-16 pointer-events-none">
+        <div className="text-center pointer-events-auto">
+          <h1 className="text-xl font-semibold text-neutral-700 mb-1">访客 Widget 预览</h1>
+          <p className="text-sm text-neutral-400">右下角为可嵌入聊天组件 · 消息会写入数据库</p>
         </div>
       </div>
-      {showWidget && <VisitorChat />}
+      <VisitorChat defaultOpen />
     </div>
   )
 }