feat: add M4A metadata/cover embed support across all Flutter screens

Add FFmpegService.embedMetadataToM4a() for writing tags and cover art
into M4A files via FFmpeg. Fix two bugs in the same function:
- Remove '-disposition:v:0 attached_pic' which is only valid for
  Matroska/WebM containers and causes FFmpeg to error on MP4/M4A
- Apply same fix to _convertToAlac which had the identical issue

Add M4A handling (isM4A branch) to all four embed call-sites:
track_metadata_screen (lyrics embed, re-enrich, edit metadata sheet,
format conversion), queue_tab, local_album_screen, and
downloaded_album_screen.

Add 'LYRICS'/'UNSYNCEDLYRICS' to _mapMetadataForTagEmbed so existing
lyrics survive a re-enrich cycle on M4A/MP3/Opus files.

Preserve existing lyrics before overwriting tags in the edit metadata
sheet (best-effort readFileMetadata before FFmpeg pass).

Extract mergePlatformMetadataForTagEmbed() into lyrics_metadata_helper
to deduplicate the identical metadata-mapping loops that existed in
queue_tab, local_album_screen, downloaded_album_screen, and
track_metadata_screen.

Wire ensureLyricsMetadataForConversion into the format conversion path
in track_metadata_screen so lyrics are carried through conversions.

Add ISRC and LABEL/ORGANIZATION mappings to _convertToM4aTags.

lib/screens/queue_tab.dart

@@ -4400,6 +4400,11 @@ class _QueueTabState extends ConsumerState<QueueTab> {
     final format = item.format?.toLowerCase();
     final lowerPath = item.filePath.toLowerCase();
     final isMp3 = format == 'mp3' || lowerPath.endsWith('.mp3');
+    final isM4A =
+        format == 'm4a' ||
+        format == 'aac' ||
+        lowerPath.endsWith('.m4a') ||
+        lowerPath.endsWith('.aac');
     final isOpus =
         format == 'opus' ||
         format == 'ogg' ||
@@ -4413,6 +4418,12 @@ class _QueueTabState extends ConsumerState<QueueTab> {
         coverPath: effectiveCoverPath,
         metadata: metadata,
       );
+    } else if (isM4A) {
+      ffmpegResult = await FFmpegService.embedMetadataToM4a(
+        m4aPath: ffmpegTarget,
+        coverPath: effectiveCoverPath,
+        metadata: metadata,
+      );
     } else if (isOpus) {
       ffmpegResult = await FFmpegService.embedMetadataToOpus(
         opusPath: ffmpegTarget,
@@ -5090,12 +5101,7 @@ class _QueueTabState extends ConsumerState<QueueTab> {
         try {
           final result = await PlatformBridge.readFileMetadata(item.filePath);
           if (result['error'] == null) {
-            result.forEach((key, value) {
-              if (key == 'error' || value == null) return;
-              final v = value.toString().trim();
-              if (v.isEmpty) return;
-              metadata[key.toUpperCase()] = v;
-            });
+            mergePlatformMetadataForTagEmbed(target: metadata, source: result);
           }
         } catch (_) {}
         await ensureLyricsMetadataForConversion(
@@ -5473,7 +5479,8 @@ class _QueueTabState extends ConsumerState<QueueTab> {
                         icon: Icons.download_for_offline_outlined,
                         label:
                             '${context.l10n.queueFlacAction} ($flacEligibleCount)',
-                        onPressed: () => _queueSelectedLocalAsFlac(unifiedItems),
+                        onPressed: () =>
+                            _queueSelectedLocalAsFlac(unifiedItems),
                         colorScheme: colorScheme,
                       ),
                     ),