[BUGFIX] Correct zindex on scaffold on mobile

Corrects z-index to not hide module menu and other
important actions behind the modal backdrop on mobile

Resolves: #104933
Releases: main
Change-Id: I3d6fe35f811a01ee674adae1a1acf7c79d6edf34
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/86045


Tested-by: Andreas Nedbal <andy@pixelde.su>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>
Reviewed-by: Andreas Nedbal <andy@pixelde.su>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>