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/register/register.json

@@ -0,0 +1,4 @@
+{
+  "navigationBarTitleText": "完善资料",
+  "navigationStyle": "custom"
+}