Open a URL with auth

Use openWithAuth(url:) to sign users in automatically when they open a Setapp link. For example, when a user opens the AI credits purchase page, they land signed in and can buy credits right away. Without it, they'd have to sign in first.

SetappManager.shared.openWithAuth(url: url)

Note: Only URLs on the Setapp domain are accepted. Passing any other URL is treated as a programmer error and crashes the app. If the user's credentials can't be fetched, the page still opens, but unauthorized.