修复：管理员默认商户与账户设置

backend/internal/service/merchant_test.go

@@ -227,7 +227,7 @@ func TestUserServiceCannotDeleteOrDisableLastAdmin(t *testing.T) {
 	}
 }
 
-func TestPlatformAdminCRUDDoesNotCreateMerchantMembership(t *testing.T) {
+func TestPlatformAdminCRUDUsesHiddenSelfMerchantContext(t *testing.T) {
 	db := newServiceTestDB(t)
 	svc := NewUserService(db, NewTenantService(db))
 	actor, err := svc.CreatePlatformAdmin("existing-admin", "password123", "现有管理员")
@@ -242,8 +242,15 @@ func TestPlatformAdminCRUDDoesNotCreateMerchantMembership(t *testing.T) {
 	if err := db.Model(&model.MerchantMember{}).Where("user_id = ?", admin.ID).Count(&membershipCount).Error; err != nil {
 		t.Fatalf("count merchant memberships: %v", err)
 	}
-	if membershipCount != 0 {
-		t.Fatalf("platform admin must not have merchant memberships, count=%d", membershipCount)
+	if membershipCount != 1 {
+		t.Fatalf("platform admin should have one hidden default membership, count=%d", membershipCount)
+	}
+	var membership model.MerchantMember
+	if err := db.Where("user_id = ?", admin.ID).First(&membership).Error; err != nil {
+		t.Fatalf("load default membership: %v", err)
+	}
+	if membership.Role != model.MemberRoleOwner || !membership.IsDefault {
+		t.Fatalf("unexpected default membership: %+v", membership)
 	}
 	updated, err := svc.UpdatePlatformAdmin(admin.ID, UpdatePlatformAdminInput{
 		Username: "platform-admin-updated", Nickname: "更新后管理员", Password: "new-password", Status: 1,
@@ -266,6 +273,36 @@ func TestPlatformAdminCRUDDoesNotCreateMerchantMembership(t *testing.T) {
 	}
 }
 
+func TestUpdateCurrentAccountCanChangeUsernameNicknameAndPassword(t *testing.T) {
+	db := newServiceTestDB(t)
+	tenant := NewTenantService(db)
+	auth := NewAuthService(db, nil, tenant)
+	user, err := NewUserService(db, tenant).CreatePlatformAdmin("current-account", "old-password", "旧昵称")
+	if err != nil {
+		t.Fatalf("create account: %v", err)
+	}
+	updated, err := auth.UpdateCurrentAccount(user.ID, UpdateCurrentAccountInput{
+		CurrentPassword: "old-password",
+		Username:        "updated-account",
+		Nickname:        "新昵称",
+		NewPassword:     "new-password",
+	})
+	if err != nil {
+		t.Fatalf("update current account: %v", err)
+	}
+	if updated.Username != "updated-account" || updated.Nickname != "新昵称" {
+		t.Fatalf("unexpected updated account: %+v", updated)
+	}
+	if err := bcrypt.CompareHashAndPassword([]byte(updated.PasswordHash), []byte("new-password")); err != nil {
+		t.Fatalf("updated password should match: %v", err)
+	}
+	if _, err := auth.UpdateCurrentAccount(user.ID, UpdateCurrentAccountInput{
+		CurrentPassword: "wrong-password", Username: "should-not-save", Nickname: "", NewPassword: "",
+	}); err == nil {
+		t.Fatal("expected current password verification error")
+	}
+}
+
 func TestListMerchantAccountGroupsExcludesPlatformAdmins(t *testing.T) {
 	db := newServiceTestDB(t)
 	merchantA := model.Merchant{Code: "merchant-accounts-a", Name: "商户账号 A", Status: model.MerchantStatusActive}