OK, I feel a bit of a fool, but how does one play a card?
On PC. I'm in the combat tutorial. Lexi told me to use Strike, so I used the mouse to hover over the first Strike card and left-clicked. The card vanished, I got a 'ting!' sound and damage was recorded on the werewolf. Cool.
Lexi says to use Strike again. I hover over the remaining Strike card and left-click. Nothing happens. The mouse cursor has changed to a solid triangle with a question mark to the lower right, but neither left nor right click accomplish anything. Most of the interface points on the screen are non-responsive (probably because it's the tutorial and we haven't got to them yet).