The TestFlight link lets you download the latest development version. Check the version notes for current progress. Test builds generally carry some risk, you might find other options, I only know about this one, as signers seem a relatively new topic in Nostr development.

I think nostr:npub1xtscya34g58tk0z605fvr788k263gsu6cy9x0mhnm87echrgufzsevkk5s mentioned something about the possibility of signer functionality in Damus a while ago, perhaps the Purple version has something available.