How to freeze the z axis while rotating

I'm trying to make a third person camera but when rotating the z axis it makes it look weird and i don't know how to make it not rotate the z axis here's my code using System.Collections; using System.Collections.Generic; using UnityEngine;

 public class CameraController : MonoBehaviour
 {
 
     private GameObject player;
     private GameObject cam;
 
     public float senstivity;
     public float smoothing;
 
     // Use this for initialization
     void Start()
     {
         cam = GameObject.FindGameObjectWithTag("MainCamera");
         player = GameObject.FindGameObjectWithTag("Player");
     }
 
     // Update is called once per frame
     void Update()
     {
         if (player == null) return;
         transform.position = new Vector3(player.transform.position.x, player.transform.position.y + 4, player.transform.position.z);
 
         var mouse = new Vector2(Input.GetAxisRaw("Mouse X"), Input.GetAxisRaw("Mouse Y"));
         var v = new Vector3(-mouse.y, mouse.x, 0) * senstivity * (1 / smoothing);
         transform.localRotation *= Quaternion.Euler(v.x, v.y, 0);
     }
 }

the camera is a child of an gameobject called camera controller which stays on top of the player (because i want the camera to rotate around a point on top of the player)