fix(native-worker): preserve requested output container in finalizer

When the native worker result advertises a requested non-FLAC output extension (for example '.m4a'), skip the m4a-to-flac container conversion in both the Dart and Kotlin finalizers so the native output container is preserved end-to-end.

- ffmpeg_service: propagate the top-level 'output_extension' hint into the download-result descriptor for both the map-backed and legacy paths; expose a normalized getter for consistent comparisons.

- download_queue_provider: short-circuit the native-worker container-conversion step when the descriptor's requested extension is not '.flac', with a debug log describing the skip.

- NativeDownloadFinalizer: mirror the guard on the Kotlin side so the finalizer does not force a container conversion that would clobber the requested native output.

android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt

@@ -501,6 +501,8 @@ object NativeDownloadFinalizer {
         shouldCancel: () -> Boolean,
     ) {
         if (requestQuality(input) == "HIGH" || outputExt(input) != ".flac") return
+        val requestedDecryptionExt = requestedDecryptionOutputExt(input)
+        if (requestedDecryptionExt.isNotBlank() && requestedDecryptionExt != ".flac") return
         if (!looksLikeM4a(state.filePath, state.fileName) && !shouldForceContainerConversion(input, state)) return
 
         val localInput = materializeForFFmpeg(context, input, state)
@@ -1330,6 +1332,14 @@ object NativeDownloadFinalizer {
         return container == "m4a" || container == "mp4" || container == "mov" || container == "aac"
     }
 
+    private fun requestedDecryptionOutputExt(input: FinalizeInput): String {
+        val descriptor = input.result.optJSONObject("decryption")
+        return normalizeExt(
+            descriptor?.optString("output_extension", "")
+                ?.ifBlank { input.result.optString("output_extension", "") }
+        )
+    }
+
     private fun validateRequestContract(request: JSONObject) {
         val version = request.optInt("contract_version", -1)
         if (version != NATIVE_WORKER_CONTRACT_VERSION) {

lib/providers/download_queue_provider.dart

@@ -6049,6 +6049,16 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
     if (context.quality == 'HIGH' || context.outputExt != '.flac') {
       return filePath;
     }
+    final requestedDecryptionExt =
+        DownloadDecryptionDescriptor.fromDownloadResult(
+          result,
+        )?.normalizedOutputExtension;
+    if (requestedDecryptionExt != null && requestedDecryptionExt != '.flac') {
+      _log.d(
+        'Native-worker decrypted output requested $requestedDecryptionExt; preserving native container.',
+      );
+      return filePath;
+    }
     final lowerPath = filePath.toLowerCase();
     final resultFileName = (result['file_name'] as String?)?.toLowerCase();
     final looksLikeM4a =

lib/services/ffmpeg_service.dart

@@ -66,9 +66,9 @@ class DownloadDecryptionDescriptor {
   ) {
     final rawDecryption = result['decryption'];
     if (rawDecryption is Map) {
-      final descriptor = DownloadDecryptionDescriptor.fromJson(
-        Map<String, dynamic>.from(rawDecryption),
-      );
+      final descriptorJson = Map<String, dynamic>.from(rawDecryption);
+      descriptorJson['output_extension'] ??= result['output_extension'];
+      final descriptor = DownloadDecryptionDescriptor.fromJson(descriptorJson);
       if (descriptor.normalizedStrategy == 'ffmpeg.mov_key' &&
           descriptor.key.isNotEmpty) {
         return descriptor;
@@ -84,6 +84,7 @@ class DownloadDecryptionDescriptor {
       strategy: 'ffmpeg.mov_key',
       key: legacyKey,
       inputFormat: 'mov',
+      outputExtension: (result['output_extension'] as String?)?.trim(),
     );
   }
 
@@ -100,6 +101,12 @@ class DownloadDecryptionDescriptor {
         return strategy.trim();
     }
   }
+
+  String? get normalizedOutputExtension {
+    final trimmed = (outputExtension ?? '').trim().toLowerCase();
+    if (trimmed.isEmpty) return null;
+    return trimmed.startsWith('.') ? trimmed : '.$trimmed';
+  }
 }
 
 class FFmpegService {