Rephrase library deletion mesaging

[aaronleopold]

The messaging in the UI is not overly descriptive about what the deletion will do. Add a remark about files remaining untouched.

Discussed in #523

^{Originally posted by kulak December 14, 2024}
I see this option and I am guessing that this will delete Library AND book files. Or will it only delete Library and keep files untouched?

This option shall clearly describe the impact.

I don't think it is a good idea to remove files represented by library, because what if user decides to just re-create library with a different scan option or make some other configuration changes to the library. It would be a very nasty surprise that book files are gone.