Allow configuring to default to a particular instance

**Is your feature request related to a problem? Please describe.**

I host several GoToSocial servers. Since GTS doesn't include a frontend, I host Phanpy at `/client` and put a link there in my instance description. Users then have to hit log in, type in the URL of the instance, and log in. This is not great UX.

**Describe the solution you'd like**
It would be much more convenient if, upon clicking through, users were presented with a button to log in to that instance. Even better would be if I could link to a route that immediately redirected to the login page for my instance, so it would function as a true login button.

In other words, rather than:

Front page -> Phanpy front page -> Enter instance name -> Log in form -> Phanpy interface

I'd like to be able to allow users to do

Front page -> Log in form -> Phanpy interface

Start by tracing the existing login flow from the Phanpy front page to the login form and Mastodon instance entry. Identify the configuration or route entry points that control this flow. Done means users can open a link that presents or directly reaches the login form for a configured instance, then continue to the Phanpy interface.

Written by the indexing model from the issue text.