新增访客聊天Widget组件、Admin端布局框架及路由

web/src/pages/WidgetPreview.tsx

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+import VisitorChat from '@/widgets/VisitorChat'
+
+const WidgetPreview = () => {
+  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">访客聊天 Widget 已嵌入右下角，点击气泡按钮开始对话</p>
+        </div>
+      </div>
+      <VisitorChat />
+    </div>
+  )
+}
+
+export default WidgetPreview

web/src/pages/admin/Dashboard.tsx

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+const AdminDashboard = () => (
+  <div className="h-full flex items-center justify-center text-neutral-400 text-center">
+    <div><div className="text-lg mb-1">运营概览</div><div className="text-sm">开发中...</div></div>
+  </div>
+)
+export default AdminDashboard

web/src/pages/admin/Ops.tsx

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+const Ops = () => (
+  <div className="h-full flex items-center justify-center text-neutral-400 text-center">
+    <div><div className="text-lg mb-1">系统运维</div><div className="text-sm">开发中...</div></div>
+  </div>
+)
+export default Ops

web/src/pages/admin/Plans.tsx

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+const Plans = () => (
+  <div className="h-full flex items-center justify-center text-neutral-400 text-center">
+    <div><div className="text-lg mb-1">套餐管理</div><div className="text-sm">开发中...</div></div>
+  </div>
+)
+export default Plans

web/src/pages/admin/Tenants.tsx

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+const Tenants = () => (
+  <div className="h-full flex items-center justify-center text-neutral-400 text-center">
+    <div><div className="text-lg mb-1">租户管理</div><div className="text-sm">开发中...</div></div>
+  </div>
+)
+export default Tenants