fix: 审查修补前端布局重构并归档 change

- 补实现 ProtectedRoute 空壳组件（预留接口，不启用认证逻辑）
- 修复页面组件内联 style 为 CSS 类，符合样式规范
- 补充 Sidebar 菜单项激活状态测试、404 按钮可点击测试
- 回写 admin-layout spec Header 页面标题 fallback 行为
- 同步 delta specs 至主规范（admin-layout、frontend-routing、app-constants）
- 归档 refactor-frontend-layout change

src/web/pages/404/index.tsx

@@ -11,7 +11,7 @@ export function NotFoundPage() {
 
   return (
     <Space align="center" className="not-found-page" direction="vertical" size="large">
-      <ErrorCircleIcon size="64px" style={{ color: "var(--td-warning-color)" }} />
+      <ErrorCircleIcon className="not-found-icon" size="64px" />
       <h1>404</h1>
       <p>您访问的页面不存在</p>
       <Button onClick={handleGoHome} theme="primary">

src/web/pages/dashboard/index.tsx

@@ -14,7 +14,7 @@ export function DashboardPage() {
   });
 
   return (
-    <Space direction="vertical" size="large" style={{ width: "100%" }}>
+    <Space className="full-width-space" direction="vertical" size="large">
       <h2>欢迎使用 {APP.title}</h2>
       <p>在此构建你的应用。以下是 /health API 的返回数据（前后端联调示例）：</p>
       {health && <pre className="health-response">{JSON.stringify(health, null, 2)}</pre>}

src/web/pages/settings/index.tsx

@@ -2,7 +2,7 @@ import { Card, Space } from "tdesign-react";
 
 export function SettingsPage() {
   return (
-    <Space direction="vertical" size="large" style={{ width: "100%" }}>
+    <Space className="full-width-space" direction="vertical" size="large">
       <h2>系统设置</h2>
       <Card>
         <p>页面建设中...</p>

src/web/pages/users/index.tsx

@@ -2,7 +2,7 @@ import { Card, Space } from "tdesign-react";
 
 export function UsersPage() {
   return (
-    <Space direction="vertical" size="large" style={{ width: "100%" }}>
+    <Space className="full-width-space" direction="vertical" size="large">
       <h2>用户管理</h2>
       <Card>
         <p>页面建设中...</p>

src/web/routes.tsx

@@ -1,3 +1,5 @@
+import type { ReactNode } from "react";
+
 import { Route, Routes } from "react-router";
 
 import { NotFoundPage } from "./pages/404";
@@ -15,3 +17,7 @@ export function AppRoutes() {
     </Routes>
   );
 }
+
+export function ProtectedRoute({ children }: { children: ReactNode }) {
+  return children;
+}

src/web/styles.css

@@ -86,6 +86,14 @@
   color: var(--td-text-color-disabled);
 }
 
+.full-width-space {
+  width: 100%;
+}
+
+.not-found-icon {
+  color: var(--td-warning-color);
+}
+
 .tabular-nums {
   font-variant-numeric: tabular-nums;
 }