feat(replaygain): add Opus R128 gain tags

Write R128_TRACK_GAIN/R128_ALBUM_GAIN (Q7.8, ref -23 LUFS) alongside the existing REPLAYGAIN_* tags for .opus files so spec-compliant Opus players (RFC 7845) apply gain. Covers per-track and album embed during download (incl. SAF) and standalone re-scan.

lib/providers/download_queue_provider.dart

@@ -4860,6 +4860,10 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
             scannedReplayGain = rgResult;
             metadata['REPLAYGAIN_TRACK_GAIN'] = rgResult.trackGain;
             metadata['REPLAYGAIN_TRACK_PEAK'] = rgResult.trackPeak;
+            if (format == 'opus') {
+              final r128 = FFmpegService.replayGainDbToR128(rgResult.trackGain);
+              if (r128 != null) metadata['R128_TRACK_GAIN'] = r128;
+            }
             _log.d(
               'ReplayGain for $format: gain=${rgResult.trackGain}, peak=${rgResult.trackPeak}',
             );

lib/services/ffmpeg_service.dart

@@ -1240,8 +1240,22 @@ class FFmpegService {
     );
   }
 
-  /// Write album ReplayGain tags to a non-FLAC file (MP3/Opus) using FFmpeg.
-  /// Preserves all existing metadata and adds/overwrites album gain fields.
+  /// Convert a ReplayGain gain value (dB, referenced to -18 LUFS) into an Opus
+  /// R128 gain tag value (Q7.8 fixed point integer, referenced to -23 LUFS).
+  ///
+  /// Opus players read `R128_TRACK_GAIN` / `R128_ALBUM_GAIN` per RFC 7845, not
+  /// the `REPLAYGAIN_*` dB strings. The reference levels differ by exactly 5 dB
+  /// (-18 vs -23 LUFS), so the R128 gain equals the ReplayGain value minus 5 dB,
+  /// stored as `round(dB * 256)`.
+  static String? replayGainDbToR128(String replayGainDb) {
+    final match = RegExp(r'-?\d+\.?\d*').firstMatch(replayGainDb);
+    if (match == null) return null;
+    final rgDb = double.tryParse(match.group(0) ?? '');
+    if (rgDb == null) return null;
+    final r128Db = rgDb - 5.0;
+    return (r128Db * 256).round().toString();
+  }
+
   /// Write album ReplayGain tags to a file via FFmpeg.
   ///
   /// For local files, replaces the file in-place and returns `true`.
@@ -1277,10 +1291,21 @@ class FFmpegService {
       'REPLAYGAIN_ALBUM_GAIN=$albumGain',
       '-metadata',
       'REPLAYGAIN_ALBUM_PEAK=$albumPeak',
-      tempOutput,
-      '-y',
     ];
 
+    if (ext.toLowerCase() == '.opus') {
+      final r128 = replayGainDbToR128(albumGain);
+      if (r128 != null) {
+        arguments
+          ..add('-metadata')
+          ..add('R128_ALBUM_GAIN=$r128');
+      }
+    }
+
+    arguments
+      ..add(tempOutput)
+      ..add('-y');
+
     _log.d('Writing album ReplayGain tags via FFmpeg');
     final result = await _executeWithArguments(arguments);
 
@@ -1347,10 +1372,21 @@ class FFmpegService {
       'REPLAYGAIN_TRACK_GAIN=$trackGain',
       '-metadata',
       'REPLAYGAIN_TRACK_PEAK=$trackPeak',
-      tempOutput,
-      '-y',
     ];
 
+    if (ext.toLowerCase() == '.opus') {
+      final r128 = replayGainDbToR128(trackGain);
+      if (r128 != null) {
+        arguments
+          ..add('-metadata')
+          ..add('R128_TRACK_GAIN=$r128');
+      }
+    }
+
+    arguments
+      ..add(tempOutput)
+      ..add('-y');
+
     _log.d('Writing track ReplayGain tags via FFmpeg');
     final result = await _executeWithArguments(arguments);
 
@@ -2725,6 +2761,12 @@ class FFmpegService {
         case 'REPLAYGAINALBUMPEAK':
           vorbis['REPLAYGAIN_ALBUM_PEAK'] = value;
           break;
+        case 'R128TRACKGAIN':
+          vorbis['R128_TRACK_GAIN'] = value;
+          break;
+        case 'R128ALBUMGAIN':
+          vorbis['R128_ALBUM_GAIN'] = value;
+          break;
       }
     }