[BUGFIX] Explicitly import `Viewport` in LiveSearch

The result container in LiveSearch used to use `TYPO3.Backend` to access
the righthand module container of the TYPO3 backend scaffold, which is
bad practice as the dependency is defined somewhere. It's better to
explicitly import the `Viewport` module, providing the same
functionality.

Resolves: #100125
Releases: main
Change-Id: I139b26370739d568b190ef7375527e491086a694
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78078


Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Oliver Bartsch <bo@cedev.de>