Fix metadata consistency (Go->Flutter) and build optimization

- Backend: Return full metadata (Track, Disc, Year) from Tidal/Qobuz/Amazon download results
- Flutter: Use backend metadata for tagging converted M4A and history entries
- Fix: Duplicate convertTrack method in deezer.go
- Fix: Better error message for Deezer fallback failure
- Changed: Default service fallback to Tidal -> Qobuz -> Amazon
- Build: Re-enabled resource shrinking and minification for release build

lib/services/ffmpeg_service.dart

@@ -1,5 +1,6 @@
 import 'dart:io';
 import 'package:flutter/services.dart';
+import 'package:path_provider/path_provider.dart';
 import 'package:spotiflac_android/utils/logger.dart';
 
 final _log = AppLogger('FFmpeg');
@@ -133,22 +134,83 @@ class FFmpegService {
     }
   }
 
-  /// Embed cover art to FLAC file
+  /// Embed metadata and cover art to FLAC file
   /// Returns the file path on success, null on failure
-  static Future<String?> embedCover(String flacPath, String coverPath) async {
-    final tempOutput = '$flacPath.tmp';
-    final command = '-i "$flacPath" -i "$coverPath" -map 0:a -map 1:0 -c copy -metadata:s:v title="Album cover" -metadata:s:v comment="Cover (front)" -disposition:v attached_pic "$tempOutput" -y';
+  static Future<String?> embedMetadata({
+    required String flacPath,
+    String? coverPath,
+    Map<String, String>? metadata,
+  }) async {
+    // Android Scoped Storage: Cannot write directly to Music folder with FFmpeg
+    // Use app-internal cache directory for temp output
+    final tempDir = await getTemporaryDirectory();
+    final uniqueId = DateTime.now().millisecondsSinceEpoch;
+    final tempOutput = '${tempDir.path}/temp_embed_$uniqueId.flac';
+    
+    // Construct command
+    final StringBuffer cmdBuffer = StringBuffer();
+    cmdBuffer.write('-i "$flacPath" ');
+    
+    // Add cover input if available
+    if (coverPath != null) {
+      cmdBuffer.write('-i "$coverPath" ');
+    }
+    
+    // Map audio stream
+    cmdBuffer.write('-map 0:a ');
+    
+    // Map cover stream if available
+    if (coverPath != null) {
+      cmdBuffer.write('-map 1:0 ');
+      cmdBuffer.write('-c:v copy ');
+      cmdBuffer.write('-disposition:v attached_pic ');
+      cmdBuffer.write('-metadata:s:v title="Album cover" ');
+      cmdBuffer.write('-metadata:s:v comment="Cover (front)" ');
+    }
+    
+    // Copy audio codec (don't re-encode)
+    cmdBuffer.write('-c:a copy ');
+    
+    // Add text metadata
+    if (metadata != null) {
+      metadata.forEach((key, value) {
+        // Sanitize value: escape double quotes
+        final sanitizedValue = value.replaceAll('"', '\\"');
+        cmdBuffer.write('-metadata $key="$sanitizedValue" ');
+      });
+    }
+    
+    cmdBuffer.write('"$tempOutput" -y');
+    
+    final command = cmdBuffer.toString();
+    _log.d('Executing FFmpeg command: $command');
 
     final result = await _execute(command);
 
     if (result.success) {
       try {
-        // Replace original with temp
-        await File(flacPath).delete();
-        await File(tempOutput).rename(flacPath);
-        return flacPath;
+        // Copy temp output back to original location (replace)
+        final tempFile = File(tempOutput);
+        final originalFile = File(flacPath);
+        
+        if (await tempFile.exists()) {
+             // Delete original file
+             if (await originalFile.exists()) {
+               await originalFile.delete();
+             }
+             // Copy temp file to original location
+             await tempFile.copy(flacPath);
+             // Delete temp file
+             await tempFile.delete();
+             
+             return flacPath;
+        } else {
+             _log.e('Temp output file not found: $tempOutput');
+             return null;
+        }
+
       } catch (e) {
-        _log.e('Failed to replace file after cover embed: $e');
+        _log.e('Failed to replace file after metadata embed: $e');
         return null;
       }
     }
@@ -161,7 +223,7 @@ class FFmpegService {
       }
     } catch (_) {}
 
-    _log.e('Cover embed failed: ${result.output}');
+    _log.e('Metadata/Cover embed failed: ${result.output}');
     return null;
   }
 }