feat(player): ReplayGain/R128 volume normalization for the built-in player

Reads REPLAYGAIN_TRACK_GAIN (album gain fallback) from FLAC/MP3/M4A tags
and converts Opus R128_TRACK_GAIN/R128_ALBUM_GAIN (Q7.8, -23 LUFS) to the
same ReplayGain dB representation on the Go side, then attenuates the
internal player's volume per track. Off by default; toggle lives under
Library > Playback. Opus header output gain is left to the decoder, which
already applies it.

Closes #465

lib/models/settings.dart

@@ -20,6 +20,8 @@ class AppSettings {
   artistTagMode; // 'joined' or 'split_vorbis' for Vorbis-based formats
   final bool embedLyrics;
   final bool embedReplayGain;
+  // Apply ReplayGain/R128 tags as volume normalization in the built-in player.
+  final bool playbackNormalization;
   final bool maxQualityCover;
   final bool isFirstLaunch;
   final bool checkForUpdates;
@@ -112,6 +114,7 @@ class AppSettings {
     this.artistTagMode = artistTagModeJoined,
     this.embedLyrics = true,
     this.embedReplayGain = false,
+    this.playbackNormalization = false,
     this.maxQualityCover = true,
     this.isFirstLaunch = true,
     this.checkForUpdates = true,
@@ -179,6 +182,7 @@ class AppSettings {
     String? artistTagMode,
     bool? embedLyrics,
     bool? embedReplayGain,
+    bool? playbackNormalization,
     bool? maxQualityCover,
     bool? isFirstLaunch,
     bool? checkForUpdates,
@@ -249,6 +253,8 @@ class AppSettings {
       artistTagMode: artistTagMode ?? this.artistTagMode,
       embedLyrics: embedLyrics ?? this.embedLyrics,
       embedReplayGain: embedReplayGain ?? this.embedReplayGain,
+      playbackNormalization:
+          playbackNormalization ?? this.playbackNormalization,
       maxQualityCover: maxQualityCover ?? this.maxQualityCover,
       isFirstLaunch: isFirstLaunch ?? this.isFirstLaunch,
       checkForUpdates: checkForUpdates ?? this.checkForUpdates,

lib/models/settings.g.dart

@@ -19,6 +19,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   artistTagMode: json['artistTagMode'] as String? ?? artistTagModeJoined,
   embedLyrics: json['embedLyrics'] as bool? ?? true,
   embedReplayGain: json['embedReplayGain'] as bool? ?? false,
+  playbackNormalization: json['playbackNormalization'] as bool? ?? false,
   maxQualityCover: json['maxQualityCover'] as bool? ?? true,
   isFirstLaunch: json['isFirstLaunch'] as bool? ?? true,
   checkForUpdates: json['checkForUpdates'] as bool? ?? true,
@@ -105,6 +106,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'artistTagMode': instance.artistTagMode,
   'embedLyrics': instance.embedLyrics,
   'embedReplayGain': instance.embedReplayGain,
+  'playbackNormalization': instance.playbackNormalization,
   'maxQualityCover': instance.maxQualityCover,
   'isFirstLaunch': instance.isFirstLaunch,
   'checkForUpdates': instance.checkForUpdates,