I spent a few days on it, and just couldn't get login with Amber to work properly.

It actually is possible to do NIP-46 login to Ditto right now through the REST API (and it works perfectly with nak), but I haven't built a UI for it yet. I'm not in a rush because the signers don't work great yet. This is probably a chicken and egg problem.

So, sadly the only way on mobile is to paste the nsec, or use FireFox with a NIP-07 extension, which I'm told does actually work, but then you will have to use FireFox.