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/app.json

@@ -2,6 +2,7 @@
   "pages": [
     "pages/index/index",
     "pages/login/login",
+    "pages/register/register",
     "pages/scan/scan",
     "pages/ble-connect/ble-connect",
     "pages/bind-success/bind-success",