[feature] Signin/up Callback URLs

[SeverinAlexB]

When PubkyApp opens Ring for a sign-in/up, there is no way for Ring to go back to the application that opened it after the authorization is completed. This is a common problem on mobile devices. See https://x-callback-url.com/ .

We need to research if it's smart to add this x-callback to the signin/up flow?

Some questions that come to my mind:

• Would this work on all mobile devices?
• Will the QR code be too big? Do we need to optimize the QR code to prevent this?
• Do we need other actions like x-cancel?

So this is not just a programming task but also a feasibility/UX task.

FYI @coreyphillips @86667

[SeverinAlexB]

Related to pubky/pubky-ring#256