Hide navigation title on tvOS, move the new-playlist action into an
inline focus section above the list, and make rows focusable via
NavigationLink so focus can move down from the button and default
focus lands on the first playlist.
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.
