gameReplayLink function
Returns a shareable link to replay the finished game with gameId, or
null if appHost is not configured.
Produces a URL such as https://strategy.eigeninteractive.com/game/<id>
that opens the game on the recipient's device. A recipient who did not play
in the game lands on its replay entry; only meaningful for public games,
since the server refuses a non-participant's replay of a private one.
Implementation
Uri? gameReplayLink(String gameId, {required String? appHost}) =>
appHost != null ? Uri.https(appHost, '/game/$gameId') : null;