Fixed two bugs in album cover art resolution for multi-disc layouts:
1. compareImageFiles now sorts by path depth (shallower first) when basenames
tie, so album-root images like Artist/Album/cover.jpg are preferred over
disc-subfolder images like Artist/Album/CD1/cover.jpg.
2. commonParentFolder now includes the parent folder for single-disc-subfolder
albums, with a Path != "." guard to avoid pulling artist-folder images.
Closes#5376