feat: native M4A ReplayGain tag writing and SAF picker error handling

lib/providers/download_queue_provider.dart

@@ -3910,11 +3910,14 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
         }
       }
 
-      // ── ReplayGain (MP3/Opus: scan before FFmpeg, add to metadata) ─
+      ReplayGainResult? scannedReplayGain;
+
+      // ── ReplayGain (MP3/Opus/M4A: scan before FFmpeg, add to metadata) ─
       if (settings.embedReplayGain && !isFlac) {
         try {
           final rgResult = await FFmpegService.scanReplayGain(filePath);
           if (rgResult != null) {
+            scannedReplayGain = rgResult;
             metadata['REPLAYGAIN_TRACK_GAIN'] = rgResult.trackGain;
             metadata['REPLAYGAIN_TRACK_PEAK'] = rgResult.trackPeak;
             _log.d(
@@ -3967,6 +3970,20 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
         _log.w('FFmpeg $format metadata embed failed');
       }
 
+      if (isM4a && settings.embedReplayGain && scannedReplayGain != null) {
+        try {
+          await PlatformBridge.editFileMetadata(filePath, {
+            'replaygain_track_gain': scannedReplayGain.trackGain,
+            'replaygain_track_peak': scannedReplayGain.trackPeak,
+          });
+          _log.d(
+            'ReplayGain compatibility tags written for $format: gain=${scannedReplayGain.trackGain}, peak=${scannedReplayGain.trackPeak}',
+          );
+        } catch (e) {
+          _log.w('Failed to write native ReplayGain tags for $format: $e');
+        }
+      }
+
       // ── FLAC post-processing ────────────────────────────────────────
       if (isFlac) {
         if (settings.artistTagMode == artistTagModeSplitVorbis) {

lib/screens/setup_screen.dart

@@ -10,6 +10,9 @@ import 'package:spotiflac_android/providers/settings_provider.dart';
 import 'package:spotiflac_android/l10n/l10n.dart';
 import 'package:spotiflac_android/services/platform_bridge.dart';
 import 'package:spotiflac_android/utils/file_access.dart';
+import 'package:spotiflac_android/utils/logger.dart';
+
+final _log = AppLogger('SetupScreen');
 
 class SetupScreen extends ConsumerStatefulWidget {
   const SetupScreen({super.key});
@@ -233,7 +236,21 @@ class _SetupScreenState extends ConsumerState<SetupScreen> {
       if (Platform.isIOS) {
         await _showIOSDirectoryOptions();
       } else if (Platform.isAndroid) {
-        final result = await PlatformBridge.pickSafTree();
+        Map<String, dynamic>? result;
+        try {
+          result = await PlatformBridge.pickSafTree();
+        } catch (e) {
+          _log.w('Failed to open Android SAF picker: $e');
+          if (mounted) {
+            ScaffoldMessenger.of(context).showSnackBar(
+              SnackBar(
+                content: Text(
+                  context.l10n.snackbarCannotOpenFile(e.toString()),
+                ),
+              ),
+            );
+          }
+        }
         if (result != null) {
           final treeUri = result['tree_uri'] as String? ?? '';
           final displayName = result['display_name'] as String? ?? '';

lib/services/ffmpeg_service.dart

@@ -2169,19 +2169,6 @@ class FFmpegService {
         case 'UNSYNCEDLYRICS':
           m4aMap['lyrics'] = value;
           break;
-        // ReplayGain as iTunes freeform atoms (com.apple.iTunes:replaygain_*)
-        case 'REPLAYGAINTRACKGAIN':
-          m4aMap['REPLAYGAIN_TRACK_GAIN'] = value;
-          break;
-        case 'REPLAYGAINTRACKPEAK':
-          m4aMap['REPLAYGAIN_TRACK_PEAK'] = value;
-          break;
-        case 'REPLAYGAINALBUMGAIN':
-          m4aMap['REPLAYGAIN_ALBUM_GAIN'] = value;
-          break;
-        case 'REPLAYGAINALBUMPEAK':
-          m4aMap['REPLAYGAIN_ALBUM_PEAK'] = value;
-          break;
       }
     }