Boxdetails: show an indication that box background won't be visible if box fill is outline instead of filled

^
potential pt of confusion https://discord.com/channels/857305113936134204/857724442797146173/1391987223544594465

changed the behaviour so that if you have a background image specified but fill is empty, then the background won’t be drawn . And i’ll show an error in this scenario so the user can make their intended choice

CleanShot 2025-07-08 at 12.10.34@2x