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>

internal/store/models.go

@@ -671,6 +671,7 @@ const (
 	ActRename  = "rename"  // slave folder renamed to the master's spelling
 	ActTags    = "tags"    // tags of a file edited
 	ActMove    = "move"    // file renamed/moved by the organizer
+	ActCover   = "cover"   // album folder image written
 )
 
 // Operation statuses.