Make it clear the light/dark toggle in the group editor is only a preview

The sun / moon / auto toggle at the top of the group editor only previews how your icons look in Light and Dark Mode. It doesn't save anything, and several of you tried to set an icon's dark look from there.

Planned: a clearer "Preview" label on that toggle, and short tips the first time you open each screen.

To change an app's dark icon today: tap the app → Change Icon → Dark appearance. This doesn't affect the light icon.