Option to restrict to or default to logging into certain instance

I'm hosting Phanpy on a subdomain (phanpy.blaede.family) of my Mastodon instance (mastodon.blaede.family) to offer it as an alternate front-end. While there's not really a technical reason to restrict it to users of my instance, experience-wise it would be nice if I could easily scope it to that instance and not ask users what their instance is.

I can think a bit more about the experience/flow that would be ideal if you're curious.

Start by reviewing the current instance-selection and login flow in Phanpy, then determine how a hosted deployment could configure a default instance or restrict access to it. The issue needs a defined user flow and acceptance criteria before implementation; done should mean the configured behavior works without asking users to choose an instance.

Written by the indexing model from the issue text.