Add some `ScaleFactor` scenes to the `ui` testbed

## What problem does this solve or what need does it fill?
```
Alice 🌹 — 6:28 AM
Some stupid scale factor bug again I'm sure 🙁
I still think we should have newtyped all of this
Talin — 7:02 AM
We need a robust example with a non-1.0 scale factor to use in testing
```

The `ui` testbed example is missing any scenes that can be used to catch scale factor regressions.

## Suggested Implementation

- Create a single setup function that spawns a UI layout using every `bevy_ui` feature that can be packed into it.
- Add multiple scenes using the shared setup function, each with a different `UiScale`. Values of `0.7`, `1.`, and `1.25.` should be sufficient.
- The `UiScale` needs to be reset on despawning each scene.

## What alternative(s) have you considered?

Excessive test coverage, gratuitous newtyping.

Start with the `ui` testbed example and inspect how its existing scenes are assembled, then review the `UiScale` usage and scene despawning behavior. Add a shared setup function and scenes using scale factors `0.7`, `1.`, and `1.25`; done means the scenes exercise the available `bevy_ui` features and reset `UiScale` when despawned.

Written by the indexing model from the issue text.