Can't log into GoToSocial using Gnome Web (aka Epiphany)

### Site

phanpy.social

### Version

2025.06.22.c651e9a

### Instance

notes.kvibber.com

### Browser

- Gnome Web 48.3 (aka Epiphany)

### Bug description

On Gnome Web/Epiphany (WebKit-based), Phanpy is unable to log into my GoToSocial instance and displays "Failed to register application." when I click on the "Continue with xyz" button.

Phanpy can log into a Mastodon server on Epiphany, and it can log into the same GoToSocial instance on Vivaldi and Firefox on the same system. I've also been able to log into the same GTS instance with Phanpy on Orion on a Mac, so it's presumably not a general WebKit problem.

### To reproduce

1. On a Gnome desktop, launch the "Web" application and open phanpy.social.
2. Start to add an account.
3. Enter a GoToSocial instance and click on the button to connect.
4. See the error message!

### Expected behavior

It should open the login page for my instance.

### Other

I compared the network actions in Dev Tools on Firefox, Vivaldi and Epiphany. The error shows up immediately after the 404 response from `/.well-known/oauth-authorization-server`, which is expected since GoToSocial [doesn't implement that endpoint yet](https://codeberg.org/superseriousbusiness/gotosocial/issues/4230). On Firefox and Vivaldi, Phanpy just keeps going and load `/oauth/authorize` next, and I'm able to finish logging in. On Epiphany, it stops making network requests right after that 404, logs the 404 to the console, and shows the "Failed to register application" error in the UI.