Custom user property which is read on page load and modifies UI

[Matt]

My org is looking into the feasibility of a migration from DC to Cloud. Part of that, of course, is feature parity. We have a custom feature on DC that executes JS on every page load. I'd like to implement this in the Cloud using Forge (or whatever avenue is best). This shouldn't require adding a macro to a page.

Ideally, I'd like to take it a step further and make it a user property rather than just executed regardless. 

How to create a custom user property (an option within the user's profile)? Storage API would be my first guess, Storage API (atlassian.com), but how to I tie that into their user profile properties?

With that enabled/disabled, how do I auto load and execute the app on each page load without requiring a macro?

This is my first adventure in Confluence Cloud/Forge development so any pointers to bits and pieces of this functionality are still appreciated.

TIA!