feat(ui): increase cover art size to 600px and use CatmullRom scaling

Increased the UI cover art request size from 300px to 600px for sharper
images on high-DPI displays. Replaced BiLinear with CatmullRom (bicubic)
interpolation for higher quality image resizing. Extracted the hardcoded
size into a COVER_ART_SIZE constant in the frontend and consolidated
backend sizes into a CacheWarmerImageSizes slice. Removed the unused
UIThumbnailSize constant.

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

core/artwork/cache_warmer.go

@@ -142,15 +142,14 @@ func (a *cacheWarmer) doCacheImage(ctx context.Context, id model.ArtworkID) erro
 	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
 	defer cancel()
 
-	for _, size := range []int{consts.UICoverArtSize, consts.UIThumbnailSize} {
+	for _, size := range consts.CacheWarmerImageSizes {
 		r, _, err := a.artwork.Get(ctx, id, size, true)
 		if err != nil {
 			return fmt.Errorf("caching id='%s', size=%d: %w", id, size, err)
 		}
-		defer r.Close()
-		if _, err = io.Copy(io.Discard, r); err != nil {
-			return err
-		}
+		_, err = io.Copy(io.Discard, r)
+		r.Close()
+		return err
 	}
 	return nil
 }