Allow "mobile signer mode" where keys are ONLY stored on mobile

Feature request from a user, opening for discussion.

When syncing with mobile, we have the option of totally moving the keys off the computer and requiring all signatures to involve the phone. This may provide an improvement in security for those users. It's still not as good as true multi-sig, so it might just not make our priorities.

The issue names no files, tests, or entry points. Start by mapping the mobile sync and signing flows, then determine how keys could remain exclusively on the phone and how every signature would be requested there. Done would require an agreed design and implementation scope, since the request is explicitly open for discussion.

Written by the indexing model from the issue text.