You can download and play with my fork of cashu.me:

https://github.com/robwoodgate/cashu.me

The nsec feature is in the nsec-p2pk branch. The locking to npub feature is in the npub-p2pk branch.

Hopefully these feature branches will get pulled into the main Cashu.me wallet at some point soon.

Adding nsec directly is a bit "old school" - signers are preferred - but is generally safe if you trust the app (eg 0xchat) or it runs on local browser only (eg Cashu.me / Nostrly Cashu redeem).

At this point, due to the way signers and Cashu mints operate, you can only unlock a P2PK with the private key itself (eg nsec)