fix: avatar picker uses chooseMedia, COS domain to tx.vsai.net.cn

- Replace chooseAvatar button with chooseMedia (more reliable, works on all versions)
- COS avatar URL uses tx.vsai.net.cn instead of generated bucket domain
- Phone shows '已授权' only (no actual number displayed)

miniprogram/pages/register/register.wxss

@@ -21,21 +21,12 @@
   margin-bottom: 12rpx;
 }
 
-.avatar-btn {
-  background: none;
-  border: none;
-  padding: 0;
-  margin: 0;
-  line-height: normal;
+.avatar-wrap {
   display: flex;
   flex-direction: column;
   align-items: center;
 }
 
-.avatar-btn::after {
-  border: none;
-}
-
 .avatar-img {
   width: 160rpx;
   height: 160rpx;