ServiceWorker push notifications
If enabled (via setting), users should get notifications even when the Sapphire frontend is closed, via messages sent to a ServiceWorker.
I have noooo idea how doable this is, but hey!
We also definitely need to consider battery life, i.e. not sending notifications so often that we kill the battery of a phone that is otherwise left with its screen off, but also not making it so the user misses messages because they didn't get a notification. Dunno what the Right Thing to Do is here.