[dialog] Allow elements outside of modal dialogs to be focusable

Hey, we have a floating button that always appears above all other elements, including dialogs. Now we want to work with base-ui's dialog. Is it possible for an element outside the dialog context to remain visible and accessible (focusable)? The information [here](https://base-ui.com/react/components/dialog#placing-elements-outside-the-popup) does not help, as the button in our use case is outside the dialog in the DOM as well.

See [StackBlitz](https://stackblitz.com/edit/q3fgylv5?file=src%2FApp.tsx) attempted implementation