Path.GetPathRoot always returns "/" for absolute Unix paths, so bulk
backup preflight queried composefs root free space (often 0 on Bazzite)
instead of the filesystem containing Books/In progress.
On Unix, symlink-canonicalize paths (so /home -> /var/home) and pick the
longest DriveInfo.GetDrives() mount prefix. Route GetBackupDriveSpaces
through the shared helper so grouping and free-space queries agree.
Co-authored-by: rmcrackan <rmcrackan@gmail.com>