fix(artwork): always select the coverArt of the first disc in multi-disc albums (#3950)

* feat(artwork): sort image files for consistent cover art selection

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(artwork): improve album cover art selection by considering disc numbers

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

core/artwork/reader_album.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"path/filepath"
+	"slices"
 	"strings"
 	"time"
 
@@ -113,5 +114,10 @@ func loadAlbumFoldersPaths(ctx context.Context, ds model.DataStore, albums ...mo
 			imgFiles = append(imgFiles, filepath.Join(path, img))
 		}
 	}
+
+	// Sort image files to ensure consistent selection of cover art
+	// This prioritizes files from lower-numbered disc folders by sorting the paths
+	slices.Sort(imgFiles)
+
 	return paths, imgFiles, &updatedAt, nil
 }

core/artwork/reader_album_test.go

@@ -0,0 +1,76 @@
+package artwork
+
+import (
+	"context"
+	"path/filepath"
+	"time"
+
+	"github.com/navidrome/navidrome/model"
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Album Artwork Reader", func() {
+	Describe("loadAlbumFoldersPaths", func() {
+		var (
+			ctx        context.Context
+			ds         *fakeDataStore
+			repo       *fakeFolderRepo
+			album      model.Album
+			now        time.Time
+			expectedAt time.Time
+		)
+
+		BeforeEach(func() {
+			ctx = context.Background()
+			now = time.Now().Truncate(time.Second)
+			expectedAt = now.Add(5 * time.Minute)
+
+			// Set up the test folders with image files
+			repo = &fakeFolderRepo{
+				result: []model.Folder{
+					{
+						Path:            "Artist/Album/Disc1",
+						ImagesUpdatedAt: expectedAt,
+						ImageFiles:      []string{"cover.jpg", "back.jpg"},
+					},
+					{
+						Path:            "Artist/Album/Disc2",
+						ImagesUpdatedAt: now,
+						ImageFiles:      []string{"cover.jpg"},
+					},
+					{
+						Path:            "Artist/Album/Disc10",
+						ImagesUpdatedAt: now,
+						ImageFiles:      []string{"cover.jpg"},
+					},
+				},
+				err: nil,
+			}
+			ds = &fakeDataStore{
+				folderRepo: repo,
+			}
+			album = model.Album{
+				ID:        "album1",
+				Name:      "Album",
+				FolderIDs: []string{"folder1", "folder2", "folder3"},
+			}
+		})
+
+		It("returns sorted image files", func() {
+			_, imgFiles, imagesUpdatedAt, err := loadAlbumFoldersPaths(ctx, ds, album)
+
+			Expect(err).ToNot(HaveOccurred())
+			Expect(*imagesUpdatedAt).To(Equal(expectedAt))
+
+			// Check that image files are sorted alphabetically
+			Expect(imgFiles).To(HaveLen(4))
+
+			// The files should be sorted by full path
+			Expect(imgFiles[0]).To(Equal(filepath.FromSlash("Artist/Album/Disc1/back.jpg")))
+			Expect(imgFiles[1]).To(Equal(filepath.FromSlash("Artist/Album/Disc1/cover.jpg")))
+			Expect(imgFiles[2]).To(Equal(filepath.FromSlash("Artist/Album/Disc10/cover.jpg")))
+			Expect(imgFiles[3]).To(Equal(filepath.FromSlash("Artist/Album/Disc2/cover.jpg")))
+		})
+	})
+})