feat(client): 部门管理与员工创建增强(F32)

- 新增部门管理页：列表/创建/归档
- 用户创建支持选择部门与直属上级
- 补齐 departments/employees API 封装与导航入口

client/src/components/navigation/NavLinks.tsx

@@ -41,6 +41,11 @@ export const NAV_ITEMS: NavItem[] = [
     label: "用户管理",
     anyOf: ["system.users"],
   },
+  {
+    to: "/system/departments",
+    label: "部门管理",
+    anyOf: ["system.users"],
+  },
   { to: "/settings/profile", label: "个人设置" },
 ];