Choose two, three, or four players

As a player, I want to select the number of players
- [ ] Create an add button
- [ ] Display pieces for two players at the start
- [ ] Add pieces for another player when you click the add button, if there are already four players, remove the pieces for two players.
- [ ] Hide the add button after the first move, and display it after a reset.

No files, tests, or entry points are named. Start by locating the player-piece initialization, add-button handling, first-move behavior, and reset flow; confirm how the current two-player setup is represented. Done means selecting two to four players works, the add button follows the stated move and reset rules, and the displayed pieces match the selection.

Written by the indexing model from the issue text.