joriz-dn/Reactivities ? reverse-engineered prompt

Build me a simple full stack app called Reactivities.

I want a place where people can browse activities, see details, and create or join them from a web app. It should have a clean modern interface, basic navigation, and a backend API that stores the data properly so the app works end to end. Please include the usual things like listing items, viewing one item, and adding or updating an activity, with a few sensible sample records so it feels usable right away.

If you need to check current docs for the best way to wire things together, go ahead and look them up online. Keep the code organized and make it easy to run locally with the existing backend and client setup.