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/App.tsx

@@ -1,21 +1,33 @@
+import { Suspense, lazy } from 'react'
 import { BrowserRouter, Navigate, Route, Routes, useLocation } from 'react-router-dom'
 import { ConfigProvider, App as AntdApp, Spin } from 'antd'
 import zhCN from 'antd/locale/zh_CN'
-import { AuthProvider, useAuth } from './store/auth'
+import { AuthProvider } from './store/auth'
+import { useAuth } from './store/authContext'
 import MainLayout from './layouts/MainLayout'
 import Login from './pages/Login'
-import Dashboard from './pages/Dashboard'
-import OpenApiDocs from './pages/OpenApiDocs'
-import ApiDebugger from './pages/ApiDebugger'
-import Delivery from './pages/Delivery'
-import MerchantCenter from './pages/MerchantCenter'
-import MerchantRecharge from './pages/MerchantRecharge'
-import AlertSettings from './pages/AlertSettings'
-import PlatformMerchants from './pages/PlatformMerchants'
-import PlatformProducts from './pages/PlatformProducts'
-import PlatformUsers from './pages/PlatformUsers'
-import PlatformRock from './pages/PlatformRock'
 import type { ReactNode } from 'react'
+
+// 路由级懒加载：把原先 1.6MB 的首屏单包按页面拆分。
+// Login 与 MainLayout 保持同步加载——它们是未登录用户和登录后首帧的必经路径，
+// 懒加载只会凭空多一次往返。
+const Dashboard = lazy(() => import('./pages/Dashboard'))
+const OpenApiDocs = lazy(() => import('./pages/OpenApiDocs'))
+const ApiDebugger = lazy(() => import('./pages/ApiDebugger'))
+const Delivery = lazy(() => import('./pages/Delivery'))
+const MerchantCenter = lazy(() => import('./pages/MerchantCenter'))
+const MerchantRecharge = lazy(() => import('./pages/MerchantRecharge'))
+const AlertSettings = lazy(() => import('./pages/AlertSettings'))
+const PlatformMerchants = lazy(() => import('./pages/PlatformMerchants'))
+const PlatformProducts = lazy(() => import('./pages/PlatformProducts'))
+const PlatformUsers = lazy(() => import('./pages/PlatformUsers'))
+const PlatformRock = lazy(() => import('./pages/PlatformRock'))
+
+// RouteFallback 是懒加载页面与权限校验的统一占位，避免切换时白屏。
+function RouteFallback() {
+  return <div style={{ textAlign: 'center', padding: '100px 0' }}><Spin size="large" /></div>
+}
+
 function PrivateRoute({ children }: { children: ReactNode }) {
   const { token } = useAuth()
   if (!token) return <Navigate to="/login" replace />
@@ -41,7 +53,7 @@ function MerchantPermissionRoute({ permission, children }: { permission?: string
   const { isAdmin, merchantLoading, merchantPermissions } = useAuth()
   if (isAdmin || !permission) return <>{children}</>
   if (merchantLoading) {
-    return <div style={{ textAlign: 'center', padding: '100px 0' }}><Spin size="large" /></div>
+    return <RouteFallback />
   }
   return merchantPermissions.includes(permission) ? <>{children}</> : <Navigate to="/" replace />
 }
@@ -158,7 +170,9 @@ export default function App() {
       <AntdApp>
         <AuthProvider>
           <BrowserRouter>
-            <AppRoutes />
+            <Suspense fallback={<RouteFallback />}>
+              <AppRoutes />
+            </Suspense>
           </BrowserRouter>
         </AuthProvider>
       </AntdApp>