add format-aware tagging and replaygain fallback mapping

Write replaygain gain and peak tags from provider-specific metadata fields, then apply tagging across supported output containers with graceful cover-art fallback so non-FLAC downloads retain metadata.

internal/app/app.go

@@ -680,10 +680,8 @@ func (m *Main) ripTrack(ctx context.Context, p provider.Client, source, id, fall
 			coverPath = tag.CoverPathForTrack(outPath, opts.albumFolder)
 		}
 	}
-	if strings.EqualFold(filepath.Ext(outPath), ".flac") {
-		if err = m.Tagger.TagFLAC(outPath, tagMeta, coverPath); err != nil {
-			m.logf("warning: tag failed for %s: %v\n", filepath.Base(outPath), err)
-		}
+	if err = m.Tagger.TagFLAC(outPath, tagMeta, coverPath); err != nil {
+		m.logf("warning: tag failed for %s: %v\n", filepath.Base(outPath), err)
 	}
 
 	if m.Config.Session.Conversion.Enabled {
@@ -847,15 +845,19 @@ func titleFromMetadata(meta map[string]any, fallback string) string {
 }
 
 func nestedString(v map[string]any, keys ...string) string {
+	return stringFromAny(nestedAny(v, keys...))
+}
+
+func nestedAny(v map[string]any, keys ...string) any {
 	cur := any(v)
 	for _, key := range keys {
 		m, ok := cur.(map[string]any)
 		if !ok {
-			return ""
+			return nil
 		}
 		cur = m[key]
 	}
-	return stringFromAny(cur)
+	return cur
 }
 
 func stringFromAny(v any) string {
@@ -904,6 +906,31 @@ func boolFromAny(v any) bool {
 	return b
 }
 
+func replaygainGainFromAny(v any) string {
+	s := strings.TrimSpace(stringFromAny(v))
+	if s == "" {
+		return ""
+	}
+
+	lower := strings.ToLower(s)
+	if strings.HasSuffix(lower, "db") {
+		n := strings.TrimSpace(s[:len(s)-2])
+		if n == "" {
+			return ""
+		}
+		return n + " dB"
+	}
+
+	if _, err := strconv.ParseFloat(s, 64); err == nil {
+		return s + " dB"
+	}
+	return s
+}
+
+func replaygainPeakFromAny(v any) string {
+	return strings.TrimSpace(stringFromAny(v))
+}
+
 func trackMetaAlbum(trackMeta map[string]any) map[string]any {
 	album, ok := trackMeta["album"].(map[string]any)
 	if !ok {
@@ -967,6 +994,22 @@ func buildTagMetadata(trackMeta map[string]any, title, source, trackID string, o
 	comment := stringFromAny(trackMeta["comment"])
 	description := stringFromAny(trackMeta["description"])
 	lyrics := stringFromAny(trackMeta["lyrics"])
+	trackGain := replaygainGainFromAny(trackMeta["replaygain_track_gain"])
+	if trackGain == "" {
+		trackGain = replaygainGainFromAny(trackMeta["replayGain"])
+	}
+	albumGain := replaygainGainFromAny(trackMeta["replaygain_album_gain"])
+	if albumGain == "" {
+		albumGain = replaygainGainFromAny(nestedAny(trackMeta, "album", "replaygain_album_gain"))
+	}
+	trackPeak := replaygainPeakFromAny(trackMeta["replaygain_track_peak"])
+	if trackPeak == "" {
+		trackPeak = replaygainPeakFromAny(trackMeta["peak"])
+	}
+	albumPeak := replaygainPeakFromAny(trackMeta["replaygain_album_peak"])
+	if albumPeak == "" {
+		albumPeak = replaygainPeakFromAny(nestedAny(trackMeta, "album", "replaygain_album_peak"))
+	}
 
 	sourceAlbumID := nestedString(trackMeta, "album", "id")
 	sourceArtistID := nestedString(trackMeta, "artist", "id")
@@ -990,8 +1033,10 @@ func buildTagMetadata(trackMeta map[string]any, title, source, trackID string, o
 		Lyrics:              lyrics,
 		Copyright:           stringFromAny(trackMeta["copyright"]),
 		ISRC:                stringFromAny(trackMeta["isrc"]),
-		ReplaygainTrackGain: stringFromAny(trackMeta["replaygain_track_gain"]),
-		ReplaygainAlbumGain: stringFromAny(trackMeta["replaygain_album_gain"]),
+		ReplaygainTrackGain: trackGain,
+		ReplaygainAlbumGain: albumGain,
+		ReplaygainTrackPeak: trackPeak,
+		ReplaygainAlbumPeak: albumPeak,
 		SourcePlatform:      source,
 		SourceTrackID:       trackID,
 		SourceAlbumID:       sourceAlbumID,

internal/app/app_test.go

@@ -357,3 +357,30 @@ func TestTrackOutputPathSinglesUsesAlbumID(t *testing.T) {
 		t.Fatalf("trackOutputPath() dir=%q want %q", got, want)
 	}
 }
+
+func TestBuildTagMetadataReplayGainFallbacks(t *testing.T) {
+	meta := map[string]any{
+		"replayGain": float64(-7.25),
+		"peak":       float64(0.989),
+		"album": map[string]any{
+			"title":                 "Album",
+			"replaygain_album_gain": float64(-8.1),
+			"replaygain_album_peak": float64(1.001),
+		},
+		"performer": map[string]any{"name": "Artist"},
+	}
+
+	tags := buildTagMetadata(meta, "Song", "tidal", "t1", ripTrackOptions{})
+	if tags.ReplaygainTrackGain != "-7.25 dB" {
+		t.Fatalf("track replaygain gain=%q", tags.ReplaygainTrackGain)
+	}
+	if tags.ReplaygainAlbumGain != "-8.1 dB" {
+		t.Fatalf("album replaygain gain=%q", tags.ReplaygainAlbumGain)
+	}
+	if tags.ReplaygainTrackPeak != "0.989" {
+		t.Fatalf("track replaygain peak=%q", tags.ReplaygainTrackPeak)
+	}
+	if tags.ReplaygainAlbumPeak != "1.001" {
+		t.Fatalf("album replaygain peak=%q", tags.ReplaygainAlbumPeak)
+	}
+}