fix(admin): 修复首次进入系统后点击任意导航被拉回 dashboard

根因：RouteKeeper 保活缓存了首页 index 节点，DefaultRoute 中的声明式
<Navigate> 会随缓存节点在后续路由变化时反复触发 replace，导致所有导航
被强制拉回落地页（dashboard），刷新后因不再经过 / 而恢复正常。

修复：DefaultRoute 改为 effect 导航，仅在确实处于 / 且已算出落地页时
跳转一次；navigate 通过 ref 持有避免 effect 空转。新增集成测试覆盖
「首次进入 / → 跳转一次 → 后续导航不被劫持 → 回到 / 仍能重定向」。

apps/admin/src/components/DefaultRoute.integration.test.tsx

@@ -0,0 +1,105 @@
+import { act } from 'react';
+import { createRoot } from 'react-dom/client';
+import { MemoryRouter, Route, Routes, useNavigate } from 'react-router';
+import { afterEach, beforeAll, beforeEach, describe, expect, it } from 'vitest';
+import DefaultRoute from './DefaultRoute';
+import { RouteKeeper } from './RouteKeeper';
+import { usePermissionStore } from '../store/permission/permissionStore';
+import { useUserStore } from '../store/user/userStore';
+
+let container: HTMLDivElement | null = null;
+let root: ReturnType<typeof createRoot> | null = null;
+
+beforeAll(() => {
+  (
+    globalThis as typeof globalThis & { IS_REACT_ACT_ENVIRONMENT: boolean }
+  ).IS_REACT_ACT_ENVIRONMENT = true;
+});
+
+beforeEach(() => {
+  // 清掉可能由其他测试文件遗留的持久化数据，保证会话状态可控
+  Object.keys(localStorage).forEach((key) => localStorage.removeItem(key));
+  useUserStore.setState({ token: null, user: null });
+  usePermissionStore.setState({ permissions: [], status: 'unknown' });
+  useUserStore.getState().setSession('test-token', {
+    id: 1,
+    username: 'admin',
+    roles: ['超级管理员'],
+  });
+  usePermissionStore.getState().writePermissions(['dashboard:view', 'student:view']);
+});
+
+afterEach(async () => {
+  if (root) await act(async () => root?.unmount());
+  container?.remove();
+  root = null;
+  container = null;
+});
+
+function PageDashboard() {
+  const navigate = useNavigate();
+  return (
+    <div>
+      <div data-testid="page-dashboard">数据面板</div>
+      <button data-testid="go-students" onClick={() => navigate('/students')}>
+        去学生管理
+      </button>
+    </div>
+  );
+}
+
+function PageStudents() {
+  const navigate = useNavigate();
+  return (
+    <div>
+      <div data-testid="page-students">学生管理</div>
+      <button data-testid="go-home" onClick={() => navigate('/')}>
+        回首页
+      </button>
+    </div>
+  );
+}
+
+function Harness() {
+  return (
+    <MemoryRouter initialEntries={['/']}>
+      <Routes>
+        <Route path="/" element={<RouteKeeper />}>
+          <Route index element={<DefaultRoute />} />
+          <Route path="dashboard" element={<PageDashboard />} />
+          <Route path="students" element={<PageStudents />} />
+        </Route>
+      </Routes>
+    </MemoryRouter>
+  );
+}
+
+async function renderHarness() {
+  container = document.createElement('div');
+  document.body.appendChild(container);
+  root = createRoot(container);
+  await act(async () => {
+    root?.render(<Harness />);
+  });
+}
+
+describe('DefaultRoute keep-alive regression', () => {
+  it('redirects to landing page once and does not hijack later navigations', async () => {
+    await renderHarness();
+
+    // 首次进入 '/' 应跳转到落地页 /dashboard，且只跳一次
+    expect(document.querySelector('[data-testid="page-dashboard"]')).not.toBeNull();
+
+    // 再导航到 /students，不应被保活的首页节点拉回 /dashboard
+    await act(async () => {
+      (document.querySelector('[data-testid="go-students"]') as HTMLButtonElement).click();
+    });
+    expect(document.querySelector('[data-testid="page-students"]')).not.toBeNull();
+
+    // 回到 '/' 时仍应再次跳转到落地页
+    await act(async () => {
+      (document.querySelector('[data-testid="go-home"]') as HTMLButtonElement).click();
+    });
+    expect(document.querySelector('[data-testid="page-dashboard"]')).not.toBeNull();
+  });
+});