map deezer gain to replaygain track metadata

internal/app/app.go

@@ -1248,6 +1248,9 @@ func buildTagMetadata(trackMeta map[string]any, title, source, trackID string, o
 	if trackGain == "" {
 		trackGain = replaygainGainFromAny(trackMeta["replayGain"])
 	}
+	if trackGain == "" {
+		trackGain = replaygainGainFromAny(trackMeta["gain"])
+	}
 	albumGain := replaygainGainFromAny(trackMeta["replaygain_album_gain"])
 	if albumGain == "" {
 		albumGain = replaygainGainFromAny(nestedAny(trackMeta, "album", "replaygain_album_gain"))