Maybe you could load

GitHub

GitHub - fiatjaf/window.nostr.js: drop-in <script> that adds seamless NIP-46 support to apps that rely on window.nostr

drop-in <script> that adds seamless NIP-46 support to apps that rely on window.nostr - fiatjaf/window.nostr.js

and display it when window.nostr is not available? That also comes with an nstart link to create a Nostr account as @elsat wants.