🧒 Support handicaps

[Terkwood]

Support handicaps

Use the existing GameState model, and simply set board.pieces as needed, and player_up to white

This ticket depends on #187

How to choose

After color pref selection, if the player chose black, ask them if they want a handicap.

If both players choose black, do not allow handicaps

Multiplayer TODOs

• [ ] micro stack

Bot TODOs

• [ ] update tinybrain