Add finding and removing folders without music

The new Cleanup tab lists the top-most folders in either library that
contain no music file (per the pair's extensions). Removing them needs
confirmation, plus an explicit acknowledgement when they still hold
files; each folder is re-checked for music and moved into the
quarantine (undoable from History, purged after the retention period).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -76,6 +76,13 @@ Each row has the match status and type, a recommendation, both paths, where link
 bit depth and duration for both sides, the quality verdict and any notes. The file opens cleanly in Excel, Numbers and LibreOffice
 (UTF-8, and cells a spreadsheet would treat as formulas are escaped).
 
+**Folders without music** (the *Cleanup* tab). Choose the slave or master library and **Find folders**: playlink lists the top-most
+folders with no music file anywhere inside them (music = the pair's file extensions), such as album folders left empty after linking or
+ones holding only a cover or playlist, with what they still contain. Empty folders are pre-selected, folders that still hold files are
+not. **Remove** asks for confirmation (and an extra acknowledgement when the folders still hold files), re-checks each folder for music
+right before touching it, and moves it into the quarantine instead of deleting it: it can be restored from History until the retention
+period purges it. The library root, the quarantine and hidden folders are never listed, and dry run applies.
+
 **Backups** (Settings → Backups), similar to Sonarr. A backup is a zip of playlink's database (settings, library pairs, scan results and
 history, not music files) taken safely while the app runs. Choose a folder (default `/config/backups`), a schedule (off, every 1, 3, 7
 or 14 days) and a retention period for scheduled backups; **Back up now** makes a manual one (kept until deleted). Backups can be