Do you tip a chauffeur, and how much?

Gratuity is included in what you are quoted. You are not expected to add anything, and no chauffeur here is relying on it to make the work worth doing — they are paid properly for the job before any tip exists.

People do add something after a long day, an early start, or a driver who did something genuinely above the arrangement, and it reaches that chauffeur in full. It is welcome and it is never expected, which is the distinction worth drawing: a service where the tip is doing the work of the wage has put you in charge of someone's pay, and that is not a courtesy.

There is no screen to swipe, no suggested percentages, and nobody standing there while you decide. Tell the concierge afterwards, or add it to the ride, and it is handled without a moment at the kerb.