How can I...

Save the library of the online/offline folders?
If I reinstall serviio they are dead and gone.

[zip]

you can either just reinstall by running the installer again (windows) or overriding the files (other OSes). Or install from scratch - you'd have to keep a copy of the library folder, which might break your new installation though. Currently there is no easy way to export online sources, but there is an open ticket.

This is the part where the jew pops out of the can!
The lib folder is protected by trusted installers