feat: 重构前端为企业 Admin 后台布局，引入 React Router 路由

- 引入 React Router v7 (Declarative mode) 实现 SPA 路由
- 重构 Layout 为 Header + 侧边栏 + 内容区的企业 Admin 布局
- 新增侧边栏菜单组件，支持折叠/展开，状态持久化到 localStorage
- 新增示例页面：仪表盘、用户管理、系统设置、404
- 菜单配置与路由统一为单一数据源 (menu.tsx)
- Vite code splitting 新增 vendor-router 组
- 更新 DEVELOPMENT.md 和 README.md 文档

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

@@ -0,0 +1,24 @@
+import { screen } from "@testing-library/react";
+import { describe, expect, test } from "bun:test";
+import { createElement } from "react";
+
+import { Sidebar } from "../../../../src/web/components/Sidebar";
+import { renderWithProviders } from "../../test-utils";
+
+describe("Sidebar", () => {
+  test("渲染菜单项", () => {
+    renderWithProviders(createElement(Sidebar, { collapsed: false }));
+
+    expect(screen.getByText("仪表盘")).not.toBeNull();
+    expect(screen.getByText("用户管理")).not.toBeNull();
+    expect(screen.getByText("系统设置")).not.toBeNull();
+  });
+
+  test("折叠状态下仍渲染菜单项", () => {
+    renderWithProviders(createElement(Sidebar, { collapsed: true }));
+
+    expect(screen.getByText("仪表盘")).not.toBeNull();
+    expect(screen.getByText("用户管理")).not.toBeNull();
+    expect(screen.getByText("系统设置")).not.toBeNull();
+  });
+});