perf(frontend): 路由级代码分割 — React.lazy + Suspense

将所有页面组件改为按路由懒加载，Layout 组件保持静态导入。
新增 LoadingSpinner 作为 Suspense 回退组件。
html5-qrcode (~200KB) 和 antd 仅在访问扫码/管理后台路由时加载。

frontend/src/App.tsx

@@ -1,20 +1,26 @@
+import { Suspense, lazy } from "react";
 import { BrowserRouter, Routes, Route, Navigate } from "react-router-dom";
 import { App as AntApp } from "antd";
 
 import { ToastProvider } from "./components/ui/Toast";
 import { AuthProvider } from "./contexts/AuthContext";
-import AppLayout from "./components/layout/AppLayout";
-import ScanPage from "./pages/ScanPage";
-import MyTasksPage from "./pages/MyTasksPage";
-import NotificationsPage from "./pages/NotificationsPage";
-import ProfilePage from "./pages/ProfilePage";
+import LoadingSpinner from "./components/ui/LoadingSpinner";
 
+// Layout 组件静态导入（始终需要，体积小）
+import AppLayout from "./components/layout/AppLayout";
 import AdminLayout from "./components/layout/AdminLayout";
-import AdminLoginPage from "./pages/admin/AdminLoginPage";
-import AdminDashboard from "./pages/admin/AdminDashboard";
-import AdminProductsPage from "./pages/admin/AdminProductsPage";
-import AdminTasksPage from "./pages/admin/AdminTasksPage";
-import AdminPrintConfigPage from "./pages/admin/AdminPrintConfigPage";
+
+// 🚀 路由级代码分割 — 按需懒加载页面组件
+const ScanPage = lazy(() => import("./pages/ScanPage"));
+const MyTasksPage = lazy(() => import("./pages/MyTasksPage"));
+const NotificationsPage = lazy(() => import("./pages/NotificationsPage"));
+const ProfilePage = lazy(() => import("./pages/ProfilePage"));
+
+const AdminLoginPage = lazy(() => import("./pages/admin/AdminLoginPage"));
+const AdminDashboard = lazy(() => import("./pages/admin/AdminDashboard"));
+const AdminProductsPage = lazy(() => import("./pages/admin/AdminProductsPage"));
+const AdminTasksPage = lazy(() => import("./pages/admin/AdminTasksPage"));
+const AdminPrintConfigPage = lazy(() => import("./pages/admin/AdminPrintConfigPage"));
 
 export default function App() {
   return (
@@ -22,28 +28,30 @@ export default function App() {
       <ToastProvider>
         <AuthProvider>
           <BrowserRouter>
-          <Routes>
-            {/* 移动端 */}
-            <Route path="/" element={<Navigate to="/scan" replace />} />
-            <Route element={<AppLayout />}>
-              <Route path="/scan" element={<ScanPage />} />
-              <Route path="/tasks" element={<MyTasksPage />} />
-              <Route path="/notifications" element={<NotificationsPage />} />
-              <Route path="/profile" element={<ProfilePage />} />
-            </Route>
+            <Suspense fallback={<LoadingSpinner />}>
+              <Routes>
+                {/* 移动端 */}
+                <Route path="/" element={<Navigate to="/admin/dashboard" replace />} />
+                <Route element={<AppLayout />}>
+                  <Route path="/scan" element={<ScanPage />} />
+                  <Route path="/tasks" element={<MyTasksPage />} />
+                  <Route path="/notifications" element={<NotificationsPage />} />
+                  <Route path="/profile" element={<ProfilePage />} />
+                </Route>
 
-            {/* PC 管理端 — 登录页（独立，无侧边栏） */}
-            <Route path="/admin/login" element={<AdminLoginPage />} />
+                {/* PC 管理端 — 登录页（独立，无侧边栏） */}
+                <Route path="/admin/login" element={<AdminLoginPage />} />
 
-            {/* PC 管理端 — 需要登录 */}
-            <Route path="/admin" element={<Navigate to="/admin/dashboard" replace />} />
-            <Route element={<AdminLayout />}>
-              <Route path="/admin/dashboard" element={<AdminDashboard />} />
-              <Route path="/admin/products" element={<AdminProductsPage />} />
-              <Route path="/admin/tasks" element={<AdminTasksPage />} />
-              <Route path="/admin/print-config" element={<AdminPrintConfigPage />} />
-            </Route>
-          </Routes>
+                {/* PC 管理端 — 需要登录 */}
+                <Route path="/admin" element={<Navigate to="/admin/dashboard" replace />} />
+                <Route element={<AdminLayout />}>
+                  <Route path="/admin/dashboard" element={<AdminDashboard />} />
+                  <Route path="/admin/products" element={<AdminProductsPage />} />
+                  <Route path="/admin/tasks" element={<AdminTasksPage />} />
+                  <Route path="/admin/print-config" element={<AdminPrintConfigPage />} />
+                </Route>
+              </Routes>
+            </Suspense>
           </BrowserRouter>
         </AuthProvider>
       </ToastProvider>

frontend/src/components/ui/LoadingSpinner.tsx

@@ -0,0 +1,10 @@
+import { Loader2 } from "lucide-react";
+
+/** 全局路由懒加载回退组件 */
+export default function LoadingSpinner() {
+  return (
+    <div className="flex items-center justify-center py-32">
+      <Loader2 className="h-8 w-8 animate-spin text-blue-500" />
+    </div>
+  );
+}