Add an OptionScaffold

### What is the problem or limitation you are having?

After the introduction of Scaffolds in #4298, we'll be able to introduce an OptionScaffold, providing a way to display app-level tab content.

### Describe the solution you'd like

The API for OptionScaffold and OptionContainer should be largely similar.

On desktop, the underlying implementation will also be largely similar (i.e., a root level widget that provides "top of window" tabs).

On mobile, the underlying implementation will reflect the historical "Primary" (pre-#4297) rendering of OptionContainer. It should be able to re-use much of the implementation and tests as well.

### Describe alternatives you've considered

-

### Additional context

See discussions on #4271 for background.

Start by reading #4298 and the background discussion in #4271, then inspect the existing OptionContainer implementation and its tests. The work is complete when OptionScaffold and OptionContainer expose largely similar APIs, with desktop top-of-window tabs and mobile behavior matching the historical Primary rendering, covered by reusable tests where applicable.

Written by the indexing model from the issue text.