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_admin_rbac.py

@@ -220,3 +220,13 @@ def test_dynamic_menu_tree_preserves_parent_order_and_button_permissions():
     assert DATA_SCOPE_VALUES == {"all", "dept", "dept_and_children", "custom_dept", "self"}
     assert [item["id"] for item in tree] == ["root"]
     assert tree[0]["children"][0]["permission"] == "home:edit"
+
+
+def test_dynamic_menu_tree_keeps_granted_menu_when_parent_is_not_in_scope():
+    menus = [
+        AdminMenu(id="page", parentId="missing-parent", name="受限页面", type="page", path="/restricted", sortOrder=10, isVisible=True, isActive=True),
+    ]
+
+    tree = build_menu_tree(menus)
+
+    assert [item["id"] for item in tree] == ["page"]