Find and remove duplicate songs within the slave library

A new Duplicates filter in Review finds songs filed more than once in the
slave library, such as a single next to its album version, and recommends
which copy to keep. Removed copies go to the quarantine and can be restored.

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

README.md

@@ -76,6 +76,16 @@ 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).
 
+**Duplicates within the slave library.** The Review tab's **Duplicates** filter is separate from master/slave matching: it looks
+only inside the slave library for songs filed more than once, such as a single next to its album version, a compilation track, or one
+song in two formats. **Find duplicates** groups songs in the same top-level folder by title, using the same rules as approximate matching
+(track numbers, disc folders and release labels such as *Single Version* or *Remastered* are ignored; *Live*, *Remix*, *Acoustic* and
+similar versions are kept apart). With ffprobe, copies whose artist tags disagree are kept apart too, and each copy shows its quality and
+length, with a warning when lengths differ by more than 10 seconds. Each song has a play button to compare the copies. One copy is
+recommended to keep (the album version, then one already linked to the master, then the better quality) and the others start selected.
+**Remove** moves the selected copies into the quarantine after re-checking that each is unchanged and that another copy is still there;
+one copy of every song is always kept, emptied folders are removed, dry run applies, and every removal can be restored from History.
+
 **Listening in build mode.** Every song in the Build tab has a play button. A mini-player along the bottom of the page plays the
 folder in order, with seeking, previous/next, volume and space to pause. Songs are streamed from the master library as they are
 (nothing is converted), so they play when the browser supports the format: FLAC, MP3, AAC/M4A, Ogg/Opus and WAV in current browsers,