feat(admin): 实现动态路由菜单并修复rbac菜单树构建问题

- 修复rbac.py中菜单树构建逻辑，将`elif not menu.parentId`改为`else`，避免父菜单不存在时子菜单丢失
- 新增后端`/api/admin/system/routers`接口，作为动态路由菜单的API
- 前端新增`getRouters` API并整合到用户信息加载流程
- 添加菜单扁平化工具函数，更新路由注册逻辑以支持嵌套路由
- 新增相关测试用例并更新API和集成文档

WonderQ-Admin/tests/test_api_contracts.py

@@ -257,6 +257,13 @@ def test_admin_requires_auth_for_protected_endpoint():
     assert response.json() == {"code": 401, "msg": "请先登录后台", "data": None}
 
 
+def test_admin_dynamic_routers_requires_admin_authentication():
+    response = TestClient(create_app()).get("/api/admin/system/routers")
+
+    assert response.status_code == 401
+    assert response.json() == {"code": 401, "msg": "请先登录后台", "data": None}
+
+
 def test_admin_login_returns_token_and_user():
     admin_user = AdminUser(
         id="admin-test",