Add scaffold to yaru-master-detail page

[whiskeyPeak]

Adds a scaffold to fix indefinite issues with flutter showing the root GTK window and therefore not applying the correct theme. This is a regression from main.

maybe fixes #522

Pull request checklist

• This PR does not introduce visual changes, or
  • I ran flutter test --update-goldens and committed the changes if there were any, or
  • I added before/after/light/dark screenshots if the visual changes I made were not covered by golden tests.

    	Before	After
    Light
    Dark

[jpnurmi]

LGTM 👍

We need to take a closer look at how and where we use Scaffolds and if we could replace some of them with simpler structures but let's take it separately.