The sheet collapsed to an empty strip because a List inside a sheet
has no intrinsic height on macOS. Add an explicit minimum frame like
other sheets, gate presentationDetents to iOS, and replace the
prominent xmark toolbar button with a native Close button on macOS.
On tvOS, ContentUnavailableView inside a Group doesn't expand to fill
available space — it sizes to content and aligns top-leading. Add
.frame(maxWidth: .infinity, maxHeight: .infinity) to all instances
so they center correctly in their parent containers.
