HIPAA rules apply to covered entities, and the developer of this app does not appear to be a covered entity. If a covered entity used this service, THEY would be required to enter into a Business Associate Agreement (BAA) with the developer, at which point the developer is on the hook and HIPAA applies.
If a covered entity engages with a platform like this, without a BAA, the liability under HIPAA is borne by the Covered Entity whom the rules apply to.
That said - if you want to engage with covered entities (and I think that should be a goal) you'll need to have all your ducks in a row before they'll be interested. It's all doable though, dont let the gatekeepers push you out.
One thing I've got my eye on right now is Palantir's HealthStart initiative that seeks to streamline the compliance requirements needed to operate in this space legally. Might be worth following if you plan to take this anywhere beyond a hobby.
Last note - my statement here is only about HIPAA. There are any number of state and federal level privacy rules where liability may or may not come into play here. Have a privacy policy, follow it, protect other people's data.If you're not confident you know how to do that, find someone who is. We do have a responsibility to our users that goes well beyond our desire to learn and experiment.
Good luck!