map deezer gain to replaygain track metadata

internal/app/app_test.go

@@ -621,3 +621,16 @@ func TestBuildTagMetadataReplayGainFallbacks(t *testing.T) {
 		t.Fatalf("album replaygain peak=%q", tags.ReplaygainAlbumPeak)
 	}
 }
+
+func TestBuildTagMetadataReplayGainFallsBackToDeezerGain(t *testing.T) {
+	meta := map[string]any{
+		"gain":      float64(-10),
+		"performer": map[string]any{"name": "Artist"},
+		"album":     map[string]any{"title": "Album"},
+	}
+
+	tags := buildTagMetadata(meta, "Song", "deezer", "2675762392", ripTrackOptions{})
+	if tags.ReplaygainTrackGain != "-10 dB" {
+		t.Fatalf("track replaygain gain=%q", tags.ReplaygainTrackGain)
+	}
+}