* fix(scanner): accept absolute paths in selective scan --target
The scanner's fs.FS only accepts paths relative to the library root, so an
absolute --target path (e.g. 2:/jukebox/collection) failed with an opaque
"invalid argument" error. Rebase absolute targets onto the library root
before scanning; relative paths are unchanged.
Fixes#5943
* refactor(scanner): simplify libraryRelativePath with IsLocal and slice.ToMap
* fix(scanner): make libraryRelativePath cross-platform
Windows CI failed: the tests hardcoded Unix-style absolute paths, which are
not absolute on Windows, and filepath.Rel yields backslash-separated paths
that the io/fs-based scanner FS rejects. Build the test paths with
filepath.Abs so they are absolute on every OS, and normalize the rebased
result with filepath.ToSlash.
* fix(scanner): resolve relative library root before rebasing target
filepath.Rel cannot rebase an absolute target onto a relative library root
(e.g. the default MusicFolder=./music), so an absolute --target was left
unchanged and rejected by the rooted io/fs. Make the library root absolute
first; it resolves against the same cwd as the scanner's fs.
5a4a3099f1