feat: add ReplayGain scanning, APEv2 tag support, and fix metadata bugs

ReplayGain (track + album):
- Scan track loudness via FFmpeg ebur128 filter (-18 LUFS reference)
- Duration-weighted power-mean for album gain computation
- Support for FLAC (native Vorbis), MP3 (ID3v2 TXXX), Opus, M4A
- Album RG auto-finalizes when all album tracks complete
- Retryable gate: blocks finalization while failed/skipped items exist
- SAF support: lossy album RG writes via temp file + writeTempToSaf
- New embedReplayGain setting (off by default) with UI toggle

APEv2 tag support:
- Full APEv2 reader/writer with header+items+footer format
- Merge-based editing with override keys for explicit deletions
- Binary cover art embedding (Cover Art (Front) item)
- Library scanner support for .ape/.wv/.mpc files
- ReplayGain fields in APE read/write/edit pipeline

Bug fixes (26):
- setArtistComments wiping fields on empty string value
- APEv2 rewrite corrupting files with ID3v1 trailer
- APE edit replacing entire tag instead of merging
- ReplayGain lost on manual MP3/Opus/M4A metadata edit
- Editor metadata save losing custom tags (preserveMetadata)
- Album RG accumulator not cleaned on queue mutation
- Album gain using unweighted mean instead of power-mean
- writeAlbumReplayGainTags return value silently ignored
- SAF album RG writing to deleted temp path
- Cancelled tracks polluting album gain computation
- APE ReplayGain not wired end-to-end
- APE field deletion not working in merge
- APE cover edit was a no-op
- Album RG duplicate entries on retry
- APE apeKeysFromFields missing track/disc/lyrics mappings
- Album RG entries purged by removeItem before computation
- FFmpeg converters discarding empty metadata values
- _appendVorbisArtistEntries skipping empty value (null vs empty)
- Album RG write-back fails for SAF lossy files
- Album RG partial finalization on failed tracks
- FLAC ClearEmpty flag destroying tags on partial callers
- clearCompleted not retriggering album RG checks
- ReadFileMetadata MP3/Ogg missing label and copyright
- Cover embed on CUE split destroying split artist tags
- Album RG gain format inconsistent (missing + prefix)
- FLAC reader/editor missing tag aliases (ALBUMARTIST, LABEL, etc.)
- dart:math log shadowed by logger.dart export

lib/models/settings.dart

@@ -16,6 +16,7 @@ class AppSettings {
   final String
   artistTagMode; // 'joined' or 'split_vorbis' for Vorbis-based formats
   final bool embedLyrics;
+  final bool embedReplayGain; // Calculate and embed ReplayGain tags
   final bool maxQualityCover;
   final bool isFirstLaunch;
   final int concurrentDownloads;
@@ -90,6 +91,7 @@ class AppSettings {
     this.embedMetadata = true,
     this.artistTagMode = artistTagModeJoined,
     this.embedLyrics = true,
+    this.embedReplayGain = false,
     this.maxQualityCover = true,
     this.isFirstLaunch = true,
     this.concurrentDownloads = 1,
@@ -151,6 +153,7 @@ class AppSettings {
     bool? embedMetadata,
     String? artistTagMode,
     bool? embedLyrics,
+    bool? embedReplayGain,
     bool? maxQualityCover,
     bool? isFirstLaunch,
     int? concurrentDownloads,
@@ -207,6 +210,7 @@ class AppSettings {
       embedMetadata: embedMetadata ?? this.embedMetadata,
       artistTagMode: artistTagMode ?? this.artistTagMode,
       embedLyrics: embedLyrics ?? this.embedLyrics,
+      embedReplayGain: embedReplayGain ?? this.embedReplayGain,
       maxQualityCover: maxQualityCover ?? this.maxQualityCover,
       isFirstLaunch: isFirstLaunch ?? this.isFirstLaunch,
       concurrentDownloads: concurrentDownloads ?? this.concurrentDownloads,

lib/models/settings.g.dart

@@ -17,6 +17,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   embedMetadata: json['embedMetadata'] as bool? ?? true,
   artistTagMode: json['artistTagMode'] as String? ?? artistTagModeJoined,
   embedLyrics: json['embedLyrics'] as bool? ?? true,
+  embedReplayGain: json['embedReplayGain'] as bool? ?? false,
   maxQualityCover: json['maxQualityCover'] as bool? ?? true,
   isFirstLaunch: json['isFirstLaunch'] as bool? ?? true,
   concurrentDownloads: (json['concurrentDownloads'] as num?)?.toInt() ?? 1,
@@ -86,6 +87,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'embedMetadata': instance.embedMetadata,
   'artistTagMode': instance.artistTagMode,
   'embedLyrics': instance.embedLyrics,
+  'embedReplayGain': instance.embedReplayGain,
   'maxQualityCover': instance.maxQualityCover,
   'isFirstLaunch': instance.isFirstLaunch,
   'concurrentDownloads': instance.concurrentDownloads,