nicbarker/clay ? reverse-engineered prompt

Build me a small demo app around this Clay UI layout library that shows off how it works in a real interface.

I want a clean responsive screen with a fixed sidebar, a flexible main area, a header, cards, text that wraps nicely, and a scrollable section so I can see how layout and scrolling behave. Please make it feel polished, with a couple of simple animated transitions if that fits the library. Use Clay the way the README shows, with the single header setup and the normal begin layout, define elements, end layout flow. If there are example renderers or browser support pieces already in the repo, hook the demo up in the simplest working way and make sure it runs smoothly. Keep it easy to understand, so I can open it and immediately see how to build UI with Clay. Look up current docs online if you need to, but stay close to the project’s intended style.