功能：增加平台账号管理与管理员保护

backend/internal/service/merchant_test.go

@@ -202,3 +202,25 @@ func TestAuthServiceChangePasswordVerifiesCurrentPassword(t *testing.T) {
 		t.Fatal("old password should no longer match")
 	}
 }
+
+func TestUserServiceCannotDeleteOrDisableLastAdmin(t *testing.T) {
+	db := newServiceTestDB(t)
+	svc := NewUserService(db, nil)
+	admin, err := svc.Create("only-admin", "password123", "唯一管理员", model.RoleAdmin, 0)
+	if err != nil {
+		t.Fatalf("create admin: %v", err)
+	}
+	if err := svc.UpdateStatus(admin.ID, 0); err == nil {
+		t.Fatal("should not disable the last enabled admin")
+	}
+	other, err := svc.Create("other-admin", "password123", "另一管理员", model.RoleAdmin, 0)
+	if err != nil {
+		t.Fatalf("create second admin: %v", err)
+	}
+	if err := svc.Delete(other.ID, admin.ID); err != nil {
+		t.Fatalf("delete non-current admin: %v", err)
+	}
+	if err := svc.Delete(admin.ID, other.ID); err == nil {
+		t.Fatal("should not delete the last enabled admin")
+	}
+}