This is a cool idea! I see a second life for my old phone.
Just wondering... No server sound great, but is it? ML is get more accurate with a larger dataset. What can you realistically expect on a (limited) phone and no server?
A server has more compute power but only needed if the problem to solve requires such power. In my last company I developed a visual search system (similar to Google Image Search) and for that use cases a powerful server is required.
But for the problems this app try to solve like human detection or facial recognition, a mobile device is very capable of. In the last 2-3 years the new phones also has integrated AI chip, which make the task easier.