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

tests/web/components/Sidebar/index.test.tsx

@@ -21,4 +21,12 @@ describe("Sidebar", () => {
     expect(screen.getByText("用户管理")).not.toBeNull();
     expect(screen.getByText("系统设置")).not.toBeNull();
   });
+
+  test("高亮当前路由对应的菜单项", () => {
+    renderWithProviders(createElement(Sidebar, { collapsed: false }), { initialRoute: "/users" });
+
+    const activeItem = document.querySelector(".t-is-active");
+    expect(activeItem).not.toBeNull();
+    expect(activeItem?.textContent).toContain("用户管理");
+  });
 });

tests/web/routes/404.test.tsx

@@ -13,4 +13,12 @@ describe("NotFoundPage", () => {
     expect(screen.getByText("您访问的页面不存在")).not.toBeNull();
     expect(screen.getByText("返回首页")).not.toBeNull();
   });
+
+  test("返回首页按钮存在且可点击", () => {
+    renderWithProviders(createElement(NotFoundPage));
+
+    const button = screen.getByText("返回首页");
+    expect(button).not.toBeNull();
+    expect(button.closest("button")).not.toBeNull();
+  });
 });