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/routes/404.test.tsx

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+import { screen } from "@testing-library/react";
+import { describe, expect, test } from "bun:test";
+import { createElement } from "react";
+
+import { NotFoundPage } from "../../../src/web/pages/404";
+import { renderWithProviders } from "../test-utils";
+
+describe("NotFoundPage", () => {
+  test("渲染 404 页面", () => {
+    renderWithProviders(createElement(NotFoundPage));
+
+    expect(screen.getByText("404")).not.toBeNull();
+    expect(screen.getByText("您访问的页面不存在")).not.toBeNull();
+    expect(screen.getByText("返回首页")).not.toBeNull();
+  });
+});

tests/web/routes/dashboard.test.tsx

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+/* eslint-disable @typescript-eslint/require-await */
+import { screen } from "@testing-library/react";
+import { describe, expect, test } from "bun:test";
+import { createElement } from "react";
+
+import { DashboardPage } from "../../../src/web/pages/dashboard";
+import { renderWithProviders } from "../test-utils";
+
+describe("DashboardPage", () => {
+  test("渲染欢迎信息", () => {
+    window.fetch = (async () => {
+      return new Response(JSON.stringify({ ok: true, service: "test-app", timestamp: new Date().toISOString() }), {
+        headers: { "Content-Type": "application/json" },
+        status: 200,
+      });
+    }) as unknown as typeof fetch;
+
+    renderWithProviders(createElement(DashboardPage));
+
+    expect(screen.getByText(/欢迎使用/)).not.toBeNull();
+  });
+});

tests/web/routes/settings.test.tsx

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+import { screen } from "@testing-library/react";
+import { describe, expect, test } from "bun:test";
+import { createElement } from "react";
+
+import { SettingsPage } from "../../../src/web/pages/settings";
+import { renderWithProviders } from "../test-utils";
+
+describe("SettingsPage", () => {
+  test("渲染系统设置页面", () => {
+    renderWithProviders(createElement(SettingsPage));
+
+    expect(screen.getByText("系统设置")).not.toBeNull();
+    expect(screen.getByText("页面建设中...")).not.toBeNull();
+  });
+});

tests/web/routes/users.test.tsx

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+import { screen } from "@testing-library/react";
+import { describe, expect, test } from "bun:test";
+import { createElement } from "react";
+
+import { UsersPage } from "../../../src/web/pages/users";
+import { renderWithProviders } from "../test-utils";
+
+describe("UsersPage", () => {
+  test("渲染用户管理页面", () => {
+    renderWithProviders(createElement(UsersPage));
+
+    expect(screen.getByText("用户管理")).not.toBeNull();
+    expect(screen.getByText("页面建设中...")).not.toBeNull();
+  });
+});