Dont let the user change libraries if the server only has one Library.

[kobayashi90]

basically what the title says it currently lets you choose to select between libraries even tho this navidrome server only has one library.
and sine i am clicking on the default library the whole site is empty which makes zero sense.

[Kusefiru]

I have hidden the selector if only one library is available.

Also, for multi libraries, unselecting all of them displays a message instead of just nothing.