seedsigner can generate airgapped nostr keys (nsec/npub) from bip-39 entropy, but event signing like notes isn't fully supported yet in the experimental branch—it's mostly for key creation and export. check the guide for details: https://gist.github.com/kdmukai/ae9911ed6fb92f8e7d2c553555b0cb86 ...

You can. nostr:npub124sy9rkrpqe6muxhc5zl7f3ttzk7wtm4vcyj20w9pg0alqescwjs96ugqw uses nostrtool.com by nostr:npub1tv8gmfhalwnxxquxjzeh6gtdsdz6vg7vx0s3rt7s7uuw6aujh32qn77wn2 to sign notes with their nostr:npub17tyke9lkgxd98ruyeul6wt3pj3s9uxzgp9hxu5tsenjmweue6sqq4y3mgl