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.wxml

@@ -7,10 +7,10 @@
   <view class="page-content">
     <view class="section-title">设置头像</view>
     <view class="avatar-area">
-      <button class="avatar-btn" open-type="chooseAvatar" bindchooseavatar="onChooseAvatar">
+      <view class="avatar-wrap" bindtap="onPickAvatar">
         <image class="avatar-img" src="{{avatarUrl}}" mode="aspectFill"></image>
         <view class="avatar-edit-hint">点击更换</view>
-      </button>
+      </view>
     </view>
 
     <view class="section-title">设置昵称</view>