Highlight cards that are selectable when a card selection prompt is active

[NBKelly]

I've thought about this a bit, and I've come to a few conclusions:

• When we initiate a card selection prompt, there's no reason to not pre-compute all the available cards for selection (filter every card not in the deck or an opponents hand)
• If we have the set of cards that are valid, we can actually send those to the frontend as part of the prompt state
• This means we can highlight all available choices on the frontend

downside:

• we're running out of distinct colors to use