Is something like this possible?
- User clicks the "Login with Nostr" button
 - Input field "Enter your Nostr npub" becomes visible
 - SN sends a login/signup link via Nostr DM
 - User clicks the link
 - Logged in!
 - 🎉
 
Same as email signup/login, for this time Nostr becomes the communication platform.
signEventfunction to web apps, NIP-42 defines an event that is signed by the user to authenticate against a relay. The same type of event, or a similar one, could be used for web authentication as well.