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_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)
+	}
+}