Reorganize leaves a cover.jpg (and other leftovers) behind when it moves an album,
and the Empty Folder Cleaner is too conservative to sweep them. Two complementary
fixes over one shared 'residual file' predicate (core/library/residual_files.py:
junk + cover/scan images + lyric/metadata sidecars), so both features agree on what
a dead folder is.
1. Reorganize (proactive): _delete_album_sidecars now sweeps ALL residual files when
a source dir has no audio left — previously only a fixed list of cover NAMES, so
back.jpg / disc.png / .webp survived and kept the folder un-prunable.
2. Empty Folder Cleaner (the request): new opt-in 'Remove Residual Files' setting
(default off) treats a folder holding only images/sidecars/junk as removable —
cleans the existing backlog + arbitrary names. Auto-renders as a UI toggle.
Safe by construction: whitelist-only (a booklet.pdf / video / .txt is real content
and kept), reorganize sweep gated on no-audio, cleaner re-checks at apply time.
20 new tests; 272 reorganize/repair/empty-folder green.
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