perf(web): 路由级懒加载，修复失效 useMemo 与 HMR 全页刷新

- 11 个页面改为 React.lazy；Login 与 MainLayout 保持同步加载（未登录用户与
  登录后首帧的必经路径）；新增统一占位 RouteFallback，替换权限校验处重复的 div
  首屏静态资源 gzip 497.7 kB → 268.4 kB（-46.1%），>500 kB 体积警告消失
- PlatformUsers 的 columns/memberColumns/merchantColumns 每次渲染都重建，
  使依赖它们的 items 的 useMemo 完全失效；三个回调 useCallback 化、
  三个列定义 useMemo 化，让 memo 真正命中
- useAuth/AuthContext 拆到 store/authContext.ts，使 store/auth.tsx 只导出
  AuthProvider 组件，消除 react(only-export-components) 导致的 HMR 整页刷新
  （同步更新 7 处引用）
- MainLayout 未使用的 catch 参数清理

oxlint：5 warnings → 0 warnings / 0 errors。

frontend/src/pages/Dashboard.tsx

@@ -19,7 +19,7 @@ import { useNavigate } from 'react-router-dom'
 import { dashboardApi } from '../api'
 import type { DashboardStats } from '../types'
 import { PageHeader } from '../components/PageHeader'
-import { useAuth } from '../store/auth'
+import { useAuth } from '../store/authContext'
 
 const numberText = (value?: number) => (value ?? 0).toLocaleString('zh-CN')
 

frontend/src/pages/Login.tsx

@@ -2,7 +2,7 @@ import { useState } from 'react'
 import { useNavigate } from 'react-router-dom'
 import { Form, Input, Button, Typography, message, Space } from 'antd'
 import { UserOutlined, LockOutlined, RightOutlined } from '@ant-design/icons'
-import { useAuth } from '../store/auth'
+import { useAuth } from '../store/authContext'
 
 export default function Login() {
   const { login } = useAuth()

frontend/src/pages/MerchantCenter.tsx

@@ -13,7 +13,7 @@ import {
   message,
 } from 'antd'
 import { ReloadOutlined } from '@ant-design/icons'
-import { useAuth } from '../store/auth'
+import { useAuth } from '../store/authContext'
 import { merchantApi } from '../api'
 import { PageHeader } from '../components/PageHeader'
 import type {

frontend/src/pages/MerchantRecharge.tsx

@@ -28,7 +28,7 @@ import type { ColumnsType } from 'antd/es/table'
 import { PageHeader } from '../components/PageHeader'
 import ImageUploader from '../components/ImageUploader'
 import { merchantApi, platformApi } from '../api'
-import { useAuth } from '../store/auth'
+import { useAuth } from '../store/authContext'
 import type { PageResult, RechargeApplication } from '../types'
 import { formatDateTime } from '../utils/time'
 

frontend/src/pages/PlatformUsers.tsx

@@ -6,7 +6,7 @@ import { platformApi, userApi } from '../api'
 import { PageHeader } from '../components/PageHeader'
 import type { MerchantMember, MerchantMemberGroup, PageResult, User } from '../types'
 import { formatDateTime } from '../utils/time'
-import { useAuth } from '../store/auth'
+import { useAuth } from '../store/authContext'
 import { roleText } from './merchantCenterUtils'
 import { useNavigate } from 'react-router-dom'
 
@@ -62,11 +62,11 @@ export default function PlatformUsers() {
     setModalOpen(true)
   }
 
-  const openEdit = (record: User) => {
+  const openEdit = useCallback((record: User) => {
     setEditingAdmin(record)
     form.setFieldsValue({ username: record.username, nickname: record.nickname, password: '', status: record.status === 1 })
     setModalOpen(true)
-  }
+  }, [form])
 
   const submit = async () => {
     const values = await form.validateFields()
@@ -94,7 +94,7 @@ export default function PlatformUsers() {
     }
   }
 
-  const remove = async (record: User) => {
+  const remove = useCallback(async (record: User) => {
     try {
       await userApi.remove(record.id)
       message.success('平台管理员已删除')
@@ -102,9 +102,9 @@ export default function PlatformUsers() {
     } catch (e) {
       message.error(e instanceof Error ? e.message : '删除失败')
     }
-  }
+  }, [loadAdmins])
 
-  const removeMerchantAccount = async (record: MerchantMember) => {
+  const removeMerchantAccount = useCallback(async (record: MerchantMember) => {
     try {
       await platformApi.deleteMerchantAccount(record.user_id)
       message.success('商户账号已删除')
@@ -112,9 +112,9 @@ export default function PlatformUsers() {
     } catch (e) {
       message.error(e instanceof Error ? e.message : '删除失败')
     }
-  }
+  }, [loadMerchantAccounts])
 
-  const columns: ColumnsType<User> = [
+  const columns: ColumnsType<User> = useMemo(() => [
     { title: '用户名', dataIndex: 'username', width: 260, render: (value) => <Typography.Text strong>{value}</Typography.Text> },
     { title: '昵称', dataIndex: 'nickname', width: 220, render: (value) => value || '-' },
     { title: '状态', dataIndex: 'status', width: 120, render: (value) => value === 1 ? <Tag color="green">启用</Tag> : <Tag>停用</Tag> },
@@ -131,9 +131,9 @@ export default function PlatformUsers() {
         </Space>
       ),
     },
-  ]
+  ], [currentUser?.id, openEdit, remove])
 
-  const memberColumns: ColumnsType<MerchantMember> = [
+  const memberColumns: ColumnsType<MerchantMember> = useMemo(() => [
     { title: '员工账号', dataIndex: ['user', 'username'], width: 260, render: (_, record) => <Typography.Text strong>{record.user?.username || `#${record.user_id}`}</Typography.Text> },
     { title: '昵称', dataIndex: ['user', 'nickname'], width: 220, render: (_, record) => record.user?.nickname || '-' },
     { title: '商户角色', dataIndex: 'role', width: 160, render: (role) => <Tag color="blue">{roleText(role)}</Tag> },
@@ -150,9 +150,9 @@ export default function PlatformUsers() {
         <Button type="link" danger size="small" icon={<DeleteOutlined />}>删除账号</Button>
       </Popconfirm>,
     },
-  ]
+  ], [removeMerchantAccount])
 
-  const merchantColumns: ColumnsType<MerchantMemberGroup> = [
+  const merchantColumns: ColumnsType<MerchantMemberGroup> = useMemo(() => [
     {
       title: '商户',
       width: 320,
@@ -174,7 +174,7 @@ export default function PlatformUsers() {
         navigate('/merchant-members')
       }}>管理商户成员</Button>,
     },
-  ]
+  ], [navigate])
 
   const items = useMemo(() => [
     {