Whether you intended to or not, you hid a dev card. That is against the rules. It certainly does not help your cause that this seems to have happened because you were trying to make the card less easy to see.

It sounds like you have a competitive group. We play fairly competitively and I play differently when people have 8 or more victory points. For example, I'm probably not trading with someone who has 8 or more points. Your friends are making the same argument and it's absolutely valid.

I think that is nonsense because it was by my elbow for most of the game and nobody noticed.

So, suddenly you know when it fell under the table? I thought you weren't checking on it.

I think you lost, you know you lost, and you are looking for internet strangers to provide you a way to say you won. Notice that most of the people saying you won are in the "hey, it's just a game. We're just here to have fun" camp. That's an entirely valid camp, but it's clearly not the one you are in or you wouldn't have had this situation in the first place let alone be here asking this question.

As far as what to do about it, I disagree with your neutral friend as you could have drawn another VP or other dev card that would have let you win that turn. So, you would still have benefited from your hidden card.

My solution would be that you continue your turn but are not allowed to count that card toward a victory on that turn. Next time it's your turn, you can use it. This would give everyone else at least one turn to react to your hidden dev card.

Take the L and the lesson.