SvelteKit: prefix not supported in @auth/sveltekit/client

Environment

System:
OS: macOS 13.1
CPU: (10) arm64 Apple M1 Max
Memory: 7.74 GB / 64.00 GB
Shell: 5.8.1 - /bin/zsh
Binaries:
Node: 16.17.0 - ~/.nvm/versions/node/v16.17.0/bin/node
Yarn: 1.22.19 - ~/.nvm/versions/node/v16.17.0/bin/yarn
npm: 8.15.0 - ~/.nvm/versions/node/v16.17.0/bin/npm

Reproduction URL

https://github.com/markjaquith/sveltekit-auth-prefix-bug

Describe the issue

prefix is not fully supported. The client code from @auth/sveltekit/client for signIn and signOut have the prefix hardcoded as /auth, and thus do not work on a site where a prefix is configured.

How to reproduce

Set a prefix:

SvelteKitAuth({
		providers: [GitHub({ clientId: GITHUB_ID, clientSecret: GITHUB_SECRET })],
		prefix: '/prefix/auth',
	})

Attempt to log in using logIn from @auth/sveltekit/client. See that the browser is attempting to communicate with /auth/csrf which does not exist — should be /prefix/auth/csrf

Manually log in by going to /prefix/auth/signin.

Attempt to log out using logOut from @auth/sveltekit/client. See that the browser is attempting to communicate with /auth/csrf which does not exist — should be /prefix/auth/csrf

Expected behavior

prefix should be fully supported.

Start with the @auth/sveltekit/client implementations of signIn and signOut, then reproduce the issue using the linked sveltekit-auth-prefix-bug project with prefix set to /prefix/auth. Trace the requests to the CSRF endpoint and confirm that both client operations use the configured prefix rather than /auth.