Make Nightfall appear under "Open at Login" instead of "Allow in the Background"

[r-thomson]

It looks like the new way to accomplish “launch at login” functionality is with SMAppService instead of SMLoginItemSetEnabled, which this app uses currently.

SMAppService does not exist prior to macOS 13, though, and I’d rather not maintain two implementations of the same thing, so we’d need to drop support for older macOS versions. Maybe in a few years.