feat: add registration page for new miniprogram users

- New register page with chooseAvatar, nickname input (random default),
  and required phone number authorization via getPhoneNumber
- Login simplified: removed deprecated wx.getUserProfile, checks
  is_new_user flag to redirect new users to registration
- Server login response now includes is_new_user field

miniprogram/pages/login/login.wxss

@@ -46,26 +46,6 @@
   border: none;
 }
 
-.phone-btn {
-  display: block;
-  width: 100%;
-  padding: 28rpx;
-  background: #ffffff;
-  color: #E6508C;
-  border: 2rpx solid #E6508C;
-  border-radius: 20rpx;
-  font-size: 30rpx;
-  text-align: center;
-  margin-top: 24rpx;
-}
-
-.phone-btn::after {
-  border: none;
-}
-
-.disabled-phone-btn {
-  display: none;
-}
 
 .agreement {
   text-align: center;