fix(examples): portal select dropdowns to body

[kpal81xd]

Description

Fixes PCUI select option lists in the examples app underlapping the controls panel by rendering them as fixed overlays attached to document.body.

• Applies the overlay select to example controls and the device selector.
• Closes open option lists when the controls panel scrolls or routes clean up, so overlays do not linger.

Manual tests:
Opened #/gaussian-splatting/first-person; checked the Renderer dropdown options render above the controls panel without underlap, close on controls scroll, option selection restores cleanly, and the device selector opens as the same overlay.

Checklist

• I have read the contributing guidelines
• My code follows the project coding standards
• This PR focuses on a single change