Add album art search, auto-match and a whole-album tag editor

Search iTunes, Deezer, MusicBrainz, DuckDuckGo and Google for square covers,
embed them and/or save cover.jpg, auto-match covers for albums, and edit an
album's shared tags and track list in one place.

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

README.md

@@ -83,6 +83,25 @@ tags** buttons. Edits are safe with hardlinks:
 - Each edit is listed in History (old → new values) and can be undone; an in-place edit interrupted by a crash is rolled back on
   startup. Dry run applies.
 
+**Album editor.** Every folder in *Tags* has an **Edit album** button (like AudioShell's folder view). It shows the album fields
+once (album, album artist, year, genre, discs, composer, comment); a change there updates every song in the album, including songs in
+`Disc 01`/`Disc 02` subfolders. Below is a track grid to edit each song's disc, track number, title and artist, with **Number tracks
+in order** (fills track and disc totals too) and **Artist = album artist**. All songs are saved as one job with the same safety
+checks as above.
+
+**Album art.** The song and album editors can swap the cover. Search iTunes, Deezer, MusicBrainz (Cover Art Archive), DuckDuckGo
+or Google by artist and album or by free text; only square images are shown, and results under a minimum size are hidden. You can
+also paste an image address or upload a file. The chosen image is converted to JPEG at the saved size (1000 × 1000 by default) and
+can be embedded in the songs, saved as `cover.jpg` in the album folder, or both. OGG and Opus can't hold embedded art here, so they
+use the folder image. A replaced `cover.jpg` goes to quarantine, and every change can be undone from History.
+
+- **Auto-match art…** walks the current folder, looks up each album on iTunes, Deezer and MusicBrainz, and applies a cover only when
+  the artist and album clearly match (release suffixes like "Deluxe Edition" are ignored). Albums that already have art can be
+  skipped; albums without a confident match are listed and left untouched.
+- Sources are chosen in *Settings → Album art*. Google needs an API key with the Custom Search API and a Programmable Search Engine
+  id (cx). DuckDuckGo has no official API and may stop working. Images are downloaded by playlink itself and never from addresses on
+  your local network.
+
 **Organize (rename files from tags).** The *Organize* tab sets how files are named, in the style of Lidarr: an **artist folder**, an
 **album folder**, a **track format** and a separate **multi-disc track format**, with presets. The default is
 `{Artist Name}/{Album Title}/{track:00} {Track Title}`, and multi-disc albums get `{Medium Format} {medium:00}/…` → `Disc 01`, `Disc 02`.