⚡ update environment mode handling, refactor user registration logic, and add username/email existence checks

internal/repo/user.go

@@ -43,3 +43,19 @@ func (user *userRepo) Update(userModel *model.User) error {
 	}
 	return nil
 }
+
+func (user *userRepo) CheckUsernameExists(username string) (bool, error) {
+	var count int64
+	if err := GetDB().Model(&model.User{}).Where("username = ?", username).Count(&count).Error; err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}
+
+func (user *userRepo) CheckEmailExists(email string) (bool, error) {
+	var count int64
+	if err := GetDB().Model(&model.User{}).Where("email = ?", email).Count(&count).Error; err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}