Store data on SD card

I got a request for an option to the local copies of page scans on an external SD card.

I’ll have to do some research on how to do this on Android and iOS - if iOS supports this. It looks like iPhones don’t have SD card slots, but people sometimes use USB-C or Lightning connected storage.

At least I know on Android internal storage keeps files completely private to that app. Using external storage can make data available to other apps that have permission to access storage. Maybe there is also an app-private external storage option. If not that could be acceptable if the user is informed.

Another option (that is not mutually exclusive) is to support external storage as a backup target.