feat: add AAC lossy target and toggle for Apple Music eLRC word sync

The HIGH-quality lossy format picker can now produce an AAC/M4A 320 kbps output alongside MP3 and Opus. FFmpegService.convertM4aToLossy/convertAudioFormat, the Dart queue pipeline, the Kotlin finalizer, and the library database format helper all route .m4a through a unified aac codec path and tag the resulting file with the M4A metadata writer. The Lossy Format setting gains a new option, and the track metadata convert dialog lists AAC next to the other targets.

Apple Music lyrics gain a 'eLRC word sync' switch (default off). When disabled the pax-to-LRC formatter strips inline word timestamps, producing line-synced LRC that is safer for players that choke on eLRC; enabling it restores the previous word-by-word behaviour. The change propagates through SetLyricsFetchOptions and invalidates the global lyrics cache on toggle.

Broad l10n migration: roughly 400 previously hardcoded English strings across queue, settings, track metadata, repo, audio analysis, setup and extension screens now live in the ARB catalog, with matching plural/placeholder forms. No behaviour change beyond localisation. Existing and new unit tests (lyrics eLRC toggle and Dart settings round-trip) pass.

lib/services/ffmpeg_service.dart

@@ -295,7 +295,7 @@ class FFmpegService {
             header[0] == 0x66 && // 'f'
             header[1] == 0x4C && // 'L'
             header[2] == 0x61 && // 'a'
-            header[3] == 0x43;   // 'C'
+            header[3] == 0x43; // 'C'
       } finally {
         await raf.close();
       }
@@ -330,7 +330,8 @@ class FFmpegService {
     String? bitrate,
     bool deleteOriginal = true,
   }) async {
-    String bitrateValue = format == 'opus' ? '128k' : '320k';
+    final normalizedFormat = format.toLowerCase();
+    String bitrateValue = normalizedFormat == 'opus' ? '128k' : '320k';
     if (bitrate != null && bitrate.contains('_')) {
       final parts = bitrate.split('_');
       if (parts.length == 2) {
@@ -338,13 +339,20 @@ class FFmpegService {
       }
     }
 
-    final extension = format == 'opus' ? '.opus' : '.mp3';
+    final extension = switch (normalizedFormat) {
+      'opus' => '.opus',
+      'aac' || 'm4a' => '.m4a',
+      _ => '.mp3',
+    };
     final outputPath = _buildOutputPath(inputPath, extension);
 
     String command;
-    if (format == 'opus') {
+    if (normalizedFormat == 'opus') {
       command =
           '-v error -hide_banner -i "$inputPath" -codec:a libopus -b:a $bitrateValue -vbr on -compression_level 10 -map 0:a "$outputPath" -y';
+    } else if (normalizedFormat == 'aac' || normalizedFormat == 'm4a') {
+      command =
+          '-v error -hide_banner -i "$inputPath" -codec:a aac -b:a $bitrateValue -map 0:a -f mp4 "$outputPath" -y';
     } else {
       command =
           '-v error -hide_banner -i "$inputPath" -codec:a libmp3lame -b:a $bitrateValue -map 0:a -id3v2_version 3 "$outputPath" -y';
@@ -361,7 +369,7 @@ class FFmpegService {
       return outputPath;
     }
 
-    _log.e('M4A to $format conversion failed: ${result.output}');
+    _log.e('M4A to $normalizedFormat conversion failed: ${result.output}');
     return null;
   }
 
@@ -1839,7 +1847,7 @@ class FFmpegService {
   }
 
   /// Unified audio format conversion with full metadata + cover preservation.
-  /// Supports: FLAC/M4A/MP3/Opus -> MP3/Opus/ALAC/FLAC.
+  /// Supports: FLAC/M4A/MP3/Opus -> AAC/M4A/MP3/Opus/ALAC/FLAC.
   /// ALAC and FLAC targets are lossless (bitrate parameter is ignored).
   static Future<String?> convertAudioFormat({
     required String inputPath,
@@ -1851,7 +1859,7 @@ class FFmpegService {
     bool deleteOriginal = true,
   }) async {
     final format = targetFormat.toLowerCase();
-    if (!const {'mp3', 'opus', 'alac', 'flac'}.contains(format)) {
+    if (!const {'mp3', 'opus', 'aac', 'alac', 'flac'}.contains(format)) {
       _log.e('Unsupported target format: $targetFormat');
       return null;
     }
@@ -1874,13 +1882,20 @@ class FFmpegService {
       );
     }
 
-    final extension = format == 'opus' ? '.opus' : '.mp3';
+    final extension = switch (format) {
+      'opus' => '.opus',
+      'aac' => '.m4a',
+      _ => '.mp3',
+    };
     final outputPath = _buildOutputPath(inputPath, extension);
 
     String command;
     if (format == 'opus') {
       command =
           '-v error -hide_banner -i "$inputPath" -codec:a libopus -b:a $bitrate -vbr on -compression_level 10 -map 0:a "$outputPath" -y';
+    } else if (format == 'aac') {
+      command =
+          '-v error -hide_banner -i "$inputPath" -codec:a aac -b:a $bitrate -map 0:a -f mp4 "$outputPath" -y';
     } else {
       command =
           '-v error -hide_banner -i "$inputPath" -codec:a libmp3lame -b:a $bitrate -map 0:a -id3v2_version 3 "$outputPath" -y';
@@ -1907,6 +1922,13 @@ class FFmpegService {
           coverPath: coverPath,
           metadata: metadata,
         );
+      } else if (format == 'aac') {
+        embedResult = await embedMetadataToM4a(
+          m4aPath: outputPath,
+          coverPath: coverPath,
+          metadata: metadata,
+          preserveMetadata: true,
+        );
       } else {
         embedResult = await embedMetadataToOpus(
           opusPath: outputPath,

lib/services/library_database.dart

@@ -1753,7 +1753,9 @@ class LibraryDatabase {
         bitrate: bitrate,
       );
 
-    if (normalizedFormat == 'mp3' || normalizedFormat == 'opus') {
+    if (normalizedFormat == 'mp3' ||
+        normalizedFormat == 'opus' ||
+        normalizedFormat == 'aac') {
       updated['bitDepth'] = null;
     }
 
@@ -2014,6 +2016,8 @@ class LibraryDatabase {
     switch (targetFormat.trim().toLowerCase()) {
       case 'alac':
         return 'm4a';
+      case 'aac':
+        return 'aac';
       case 'flac':
         return 'flac';
       case 'opus':
@@ -2030,6 +2034,7 @@ class LibraryDatabase {
     switch (targetFormat.trim().toLowerCase()) {
       case 'mp3':
       case 'opus':
+      case 'aac':
         final match = RegExp(r'(\d+)').firstMatch(bitrate);
         return match != null ? int.tryParse(match.group(1)!) : null;
       default: