This is not related to correspondence game challenges, they work differently and are received through a push notification.

Here it need a persistent websocket connection with the following specifics:

• should be open when the app has focus and closed when the app is in background
• it should listen to the challenges event to be aware of new challenges

Opening a persistent websocket connection is also the way to notify other users that the player is online. Closing #404 thus.

We'll also need a challengesProvider, which is a NotifierProvider.

Upon receiving a new challenge one should:

• display a notification, using the flutter_local_notifications plugin
• have a "decline" action button in the notification (clicking on the notification would mean accepting the challenge)
• update the challengesProvider state

The challenges message always contains the current list of all challenges, so we're notified when a challenge has been cancelled. When a challenge is canceled:

• cancel the corresponding challenge notification
• update the challengesProvider state