fix(metadata): write M4A ISRC/label natively and read all edited fields from file

FFmpeg's MP4 muxer drops ISRC and label, so write them as iTunes freeform atoms natively after every M4A embed pass. The metadata screen now reads all edited fields from the file on load (file is the source of truth), fixing edited values reverting to stale cached values after reopening.

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

@@ -2643,6 +2643,19 @@ class MainActivity: FlutterFragmentActivity() {
                             }
                             result.success(response)
                         }
+                        "writeM4AFreeformTags" -> {
+                            val filePath = call.argument<String>("file_path") ?: ""
+                            val metadataJson = call.argument<String>("metadata_json") ?: "{}"
+                            val response = withContext(Dispatchers.IO) {
+                                try {
+                                    Gobackend.writeM4AFreeformTags(filePath, metadataJson)
+                                } catch (e: Exception) {
+                                    android.util.Log.e("SpotiFLAC", "writeM4AFreeformTags failed: ${e.message}", e)
+                                    """{"error":"${e.message?.replace("\"", "'")}"}"""
+                                }
+                            }
+                            result.success(response)
+                        }
                         "writeTempToSaf" -> {
                             val tempPath = call.argument<String>("temp_path") ?: ""
                             val safUri = call.argument<String>("saf_uri") ?: ""