feat(users): add user type modification functionality

- Add modifyType API endpoint in userApi
- Implement user type modification in Users.vue

src/api/index.js

@@ -50,7 +50,10 @@ export const userApi = {
         api.post('/userMgr/create', { username, passwd, type }),
     removeUser: (targetUserID) => 
         api.post('/userMgr/remove', { targetUserID }),
-    listUsers: () => api.get('/userMgr/list')
+    listUsers: () => 
+        api.get('/userMgr/list'),
+    modifyType: (userID, newType) =>
+        api.post(`/userMgr/modifyType`, { userID, newType: newType.toLowerCase() || 'visitor' })
 };
 
 export const sessionApi = {