CympH asked for screenshots with a transparent background, to drop a
plan onto their own graphics. Settings > Map layers now has a
"Transparent screenshots" toggle. With it on, the PNG leaves out the
gradient and the dot grid, so everything outside the map's floor is
see-through; the map, its layers, placed items, drawings and the page
name draw as before. The toolbar's screenshot tooltip says when it is on.

The capture shell painted the theme background under every capture
(ShadAppBuilder's ColoredBox), so the shell now paints nothing; the view
paints its own backdrop when there is one, and video frames still do.

The option is a new AppPreferences field whose default (off) reproduces
the old behavior, so preferences written before it load without a
migration; a test reads such a record. Library backups don't carry it,
like the other export preferences.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>