Add OverlayScrollbars and the React wrapper as dependencies for the Scratch editor.
The package is MIT-licensed and covers the custom horizontal scrollbar behavior we need on macOS/Chrome.
This commit only adds the dependency and wires its base styles into the standard app bundle and the web component bundle.

Wrap the Scratch iframe in OverlayScrollbars and keep the scrollbar scoped to the Scratch project view.
The bar only appears when the iframe overflows horizontally, keeps a small gutter of its own.
Shortens the iframe when visible so the scrollbar does not sit on top of the editor.
Also add focused tests around the Scratch iframe URL and scrollbar configuration.