fix: force native FLAC muxer when decrypting to .flac output

Downloads from providers that stream FLAC inside an fMP4 container (e.g. Amazon Music) were being written to disk with a .flac extension while the payload still carried ISO-BMFF atoms. The container-conversion guard then saw codec=flac and skipped the remux, leaving native FLAC tag writers to fail with 'fLaC head incorrect'.

Force '-f flac' on the decryption command whenever the target extension is .flac so FFmpeg emits a real FLAC stream, and add an 'fLaC' magic-byte probe on both the Dart and Kotlin container-conversion guards so a FLAC-in-MP4 source is remuxed rather than silently passed through as a tag-writer hazard.

lib/providers/download_queue_provider.dart

@@ -6160,9 +6160,13 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
       String? flacPath;
       try {
         final codec = await FFmpegService.probePrimaryAudioCodec(tempPath);
-        if (!FFmpegService.isLosslessAudioCodec(codec) || codec == 'flac') {
+        final isAlreadyNativeFlac =
+            codec == 'flac' && await FFmpegService.isNativeFlacFile(tempPath);
+        if (!FFmpegService.isLosslessAudioCodec(codec) || isAlreadyNativeFlac) {
           _log.d(
-            'Preserving native container; audio codec is ${codec ?? 'unknown'}, not a lossless source needing FLAC conversion.',
+            'Preserving native container; audio codec is ${codec ?? 'unknown'}'
+            '${isAlreadyNativeFlac ? ' (native FLAC)' : ''}, '
+            'no FLAC container conversion needed.',
           );
           return filePath;
         }
@@ -6203,9 +6207,13 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
     }
 
     final codec = await FFmpegService.probePrimaryAudioCodec(filePath);
-    if (!FFmpegService.isLosslessAudioCodec(codec) || codec == 'flac') {
+    final isAlreadyNativeFlac =
+        codec == 'flac' && await FFmpegService.isNativeFlacFile(filePath);
+    if (!FFmpegService.isLosslessAudioCodec(codec) || isAlreadyNativeFlac) {
       _log.d(
-        'Preserving native container; audio codec is ${codec ?? 'unknown'}, not a lossless source needing FLAC conversion.',
+        'Preserving native container; audio codec is ${codec ?? 'unknown'}'
+        '${isAlreadyNativeFlac ? ' (native FLAC)' : ''}, '
+        'no FLAC container conversion needed.',
       );
       return filePath;
     }

lib/services/ffmpeg_service.dart

@@ -283,6 +283,28 @@ class FFmpegService {
     }.contains(normalized);
   }
 
+  /// Returns `true` when [filePath] starts with the native FLAC magic bytes
+  /// (`fLaC`). Useful to distinguish a real FLAC file from a FLAC-in-MP4
+  /// container that carries a `.flac` extension or claims codec=flac.
+  static Future<bool> isNativeFlacFile(String filePath) async {
+    try {
+      final raf = await File(filePath).open();
+      try {
+        final header = await raf.read(4);
+        return header.length == 4 &&
+            header[0] == 0x66 && // 'f'
+            header[1] == 0x4C && // 'L'
+            header[2] == 0x61 && // 'a'
+            header[3] == 0x43;   // 'C'
+      } finally {
+        await raf.close();
+      }
+    } catch (e) {
+      _log.w('Native FLAC magic probe failed for $filePath: $e');
+      return false;
+    }
+  }
+
   static Future<String?> convertM4aToFlac(String inputPath) async {
     final outputPath = _buildOutputPath(inputPath, '.flac');
 
@@ -435,7 +457,16 @@ class FFmpegService {
       // Force MOV demuxer: -decryption_key is only supported by the MOV/MP4
       // demuxer. The input may carry a .flac extension (SAF mode) while actually
       // containing an encrypted M4A stream, so we must override auto-detection.
-      return '-v error -decryption_key "$key" -f $demuxerFormat -i "$inputPath" $audioMap-c copy "$outputPath" -y';
+      //
+      // When the requested output is a native .flac we also force the flac
+      // muxer (-f flac). Without it, FFmpeg infers the muxer from the output
+      // extension AND keeps the input container's stream layout, which for
+      // FLAC-in-MP4 sources would still emit an ISO-BMFF payload under a
+      // .flac filename. That file fails native FLAC tag writers later on.
+      final muxerOverride = outputPath.toLowerCase().endsWith('.flac')
+          ? '-f flac '
+          : '';
+      return '-v error -decryption_key "$key" -f $demuxerFormat -i "$inputPath" $audioMap-c copy $muxerOverride"$outputPath" -y';
     }
 
     final keyCandidates = _buildDecryptionKeyCandidates(decryptionKey);