fix: Antigravity 刷新 token 时检测 project_id 缺失

- 刷新 token 后调用 LoadCodeAssist 获取 project_id
- 如果获取失败，保留原有 project_id，标记账户为 error
- token 仍会正常更新，不影响凭证刷新
- 错误信息：账户缺少project id，可能无法使用Antigravity

backend/internal/service/admin_service.go

@@ -42,6 +42,7 @@ type AdminService interface {
 	DeleteAccount(ctx context.Context, id int64) error
 	RefreshAccountCredentials(ctx context.Context, id int64) (*Account, error)
 	ClearAccountError(ctx context.Context, id int64) (*Account, error)
+	SetAccountError(ctx context.Context, id int64, errorMsg string) error
 	SetAccountSchedulable(ctx context.Context, id int64, schedulable bool) (*Account, error)
 	BulkUpdateAccounts(ctx context.Context, input *BulkUpdateAccountsInput) (*BulkUpdateAccountsResult, error)
 
@@ -991,6 +992,10 @@ func (s *adminServiceImpl) ClearAccountError(ctx context.Context, id int64) (*Ac
 	return account, nil
 }
 
+func (s *adminServiceImpl) SetAccountError(ctx context.Context, id int64, errorMsg string) error {
+	return s.accountRepo.SetError(ctx, id, errorMsg)
+}
+
 func (s *adminServiceImpl) SetAccountSchedulable(ctx context.Context, id int64, schedulable bool) (*Account, error) {
 	if err := s.accountRepo.SetSchedulable(ctx, id, schedulable); err != nil {
 		return nil, err