修复：管理员默认商户与账户设置

backend/internal/service/user.go

@@ -7,6 +7,7 @@ import (
 
 	"golang.org/x/crypto/bcrypt"
 	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 )
 
 type UserService struct {
@@ -96,17 +97,30 @@ func (s *UserService) Create(username, password, nickname, role string, merchant
 		if err := tx.Create(user).Error; err != nil {
 			return err
 		}
+		if role == model.RoleAdmin && s.tenant != nil {
+			var selfMerchant model.Merchant
+			if err := tx.Where("code = ?", model.MerchantCodeSelfOperated).First(&selfMerchant).Error; err != nil {
+				return err
+			}
+			member := model.MerchantMember{MerchantID: selfMerchant.ID, UserID: user.ID, Role: model.MemberRoleOwner, Status: user.Status, IsDefault: true}
+			if err := tx.Clauses(clause.OnConflict{
+				Columns: []clause.Column{{Name: "merchant_id"}, {Name: "user_id"}},
+				DoUpdates: clause.Assignments(map[string]interface{}{
+					"role": model.MemberRoleOwner, "status": user.Status, "is_default": true,
+				}),
+			}).Create(&member).Error; err != nil {
+				return err
+			}
+			return nil
+		}
 		if s.tenant != nil && merchantID != 0 {
 			memberRole := model.MemberRoleOperator
-			if role == model.RoleAdmin {
-				memberRole = model.MemberRoleOwner
-			}
 			member := model.MerchantMember{
 				MerchantID: merchantID,
 				UserID:     user.ID,
 				Role:       memberRole,
 				Status:     user.Status,
-				IsDefault:  role == model.RoleAdmin,
+				IsDefault:  false,
 			}
 			if err := tx.Create(&member).Error; err != nil {
 				return err
@@ -180,8 +194,8 @@ func (s *UserService) ListMerchantAccountGroups(page, size int) ([]MerchantMembe
 	return groups, total, nil
 }
 
-// CreatePlatformAdmin creates an account with platform-only privileges. It
-// deliberately does not assign the account to any merchant.
+// CreatePlatformAdmin creates an account with platform privileges and the
+// hidden self-operated merchant context required by merchant-side routes.
 func (s *UserService) CreatePlatformAdmin(username, password, nickname string) (*model.User, error) {
 	return s.Create(username, password, nickname, model.RoleAdmin, 0)
 }