scratchfoundation/scratch-gui ? reverse-engineered prompt

Build me the Scratch 3.0 interface as a web app, so I can open it in the browser and play with the default editor screen. It should feel like the Scratch GUI, with the main project area and the controls a person would expect for creating and running projects. I want it to run locally with one simple start command, and I should be able to see it on localhost in a playground style page.

Please make it easy to develop against, and keep the code organized so it can be used as a reusable set of React components. If there are any current setup or browser compatibility details I should know, include them in the project. Also add solid tests for the main pieces, and make sure the build works cleanly. If anything in the docs or dependencies is outdated, look up the current docs online if you need to and keep the project working with modern tooling.