feat: user management - add/edit/delete users

Backend:
- Add GET /users/roles endpoint (list all roles)
- Add GET /users/depts endpoint (list all departments)

Frontend:
- Add user.ts API client (getUsers, createUser, updateUser, deleteUser, getRoles, getDepts)
- Rewrite System.vue with full user CRUD:
  - User list table with pagination
  - Add User button + dialog form
  - Edit user (username disabled, password hidden)
  - Delete user (disabled for superadmin)
  - Role and department dropdowns
  - Status radio buttons

frontend/src/api/user.ts

@@ -0,0 +1,62 @@
+import request from './request'
+
+export interface UserItem {
+  id: number
+  username: string
+  email?: string
+  real_name?: string
+  phone?: string
+  dept_id?: number
+  is_active: boolean
+  is_superuser: boolean
+  dept?: { id: number; name: string }
+  roles: { id: number; name: string; code: string }[]
+  created_at: string
+}
+
+export interface RoleItem {
+  id: number
+  name: string
+  code: string
+  description?: string
+}
+
+export interface DeptItem {
+  id: number
+  name: string
+  parent_id?: number
+  sort_order: number
+}
+
+export function getUsers(params?: { page?: number; page_size?: number; keyword?: string }) {
+  return request.get('/users', { params })
+}
+
+export function createUser(data: {
+  username: string
+  password: string
+  email?: string
+  real_name?: string
+  phone?: string
+  dept_id?: number
+  is_active?: boolean
+  role_ids?: number[]
+}) {
+  return request.post('/users', data)
+}
+
+export function updateUser(id: number, data: Partial<UserItem> & { role_ids?: number[] }) {
+  return request.put(`/users/${id}`, data)
+}
+
+export function deleteUser(id: number) {
+  return request.delete(`/users/${id}`)
+}
+
+export function getRoles() {
+  return request.get('/users/roles')
+}
+
+export function getDepts() {
+  return request.get('/users/depts')
+}