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.wxss

@@ -0,0 +1,138 @@
+.page {
+  min-height: 100vh;
+  background: #ffffff;
+}
+
+.section-title {
+  font-size: 30rpx;
+  font-weight: 600;
+  color: #333333;
+  margin-bottom: 20rpx;
+  margin-top: 36rpx;
+}
+
+.section-title:first-child {
+  margin-top: 0;
+}
+
+.avatar-area {
+  display: flex;
+  justify-content: center;
+  margin-bottom: 12rpx;
+}
+
+.avatar-btn {
+  background: none;
+  border: none;
+  padding: 0;
+  margin: 0;
+  line-height: normal;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+}
+
+.avatar-btn::after {
+  border: none;
+}
+
+.avatar-img {
+  width: 160rpx;
+  height: 160rpx;
+  border-radius: 50%;
+  border: 4rpx solid #f0f0f0;
+  background: #f5f5f5;
+}
+
+.avatar-edit-hint {
+  font-size: 24rpx;
+  color: #999999;
+  margin-top: 12rpx;
+}
+
+.input-wrap {
+  background: #f5f5f5;
+  border-radius: 16rpx;
+  padding: 24rpx 28rpx;
+  margin-bottom: 12rpx;
+}
+
+.nickname-input {
+  font-size: 30rpx;
+  color: #333333;
+  height: 48rpx;
+  line-height: 48rpx;
+}
+
+.phone-area {
+  margin-bottom: 12rpx;
+}
+
+.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;
+}
+
+.phone-btn::after {
+  border: none;
+}
+
+.phone-hint {
+  font-size: 24rpx;
+  color: #999999;
+  text-align: center;
+  margin-top: 12rpx;
+}
+
+.phone-done {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: #f0fff4;
+  border: 2rpx solid #52c41a;
+  border-radius: 20rpx;
+  padding: 28rpx;
+}
+
+.phone-icon {
+  color: #52c41a;
+  font-size: 36rpx;
+  margin-right: 16rpx;
+}
+
+.phone-text {
+  font-size: 30rpx;
+  color: #333333;
+}
+
+.submit-btn {
+  display: block;
+  width: 100%;
+  padding: 32rpx;
+  background: #E6508C;
+  color: #ffffff;
+  border: none;
+  border-radius: 20rpx;
+  font-size: 32rpx;
+  font-weight: 600;
+  text-align: center;
+  margin-top: 60rpx;
+}
+
+.submit-btn::after {
+  border: none;
+}
+
+.agreement {
+  text-align: center;
+  font-size: 24rpx;
+  color: #999999;
+  margin-top: 32rpx;
+}