ocsigen/eliom ? reverse-engineered prompt

Build me a small Eliom starter app that shows how this framework works in practice.

I want a simple OCaml web app with one home page, one extra route like /foo, and a few examples that prove the main Eliom ideas, like shared code, server code, client code, and calling the server from the browser. Please include a basic page with a button or form, a click handler, and some session based state so I can see a value change per user. If it makes sense, add one reactive example too, so the page updates without a full refresh.

Keep it beginner friendly and easy to run, with clear setup instructions and a minimal project structure. Use current Eliom and Ocsigen best practices, and look up current docs online if you need to. I mostly want something I can open, run, and learn from, not a huge app.