feat: add genre and label metadata to FLAC downloads

- Fetch genre and label from Deezer album API before download
- Add GENRE, ORGANIZATION (label), and COPYRIGHT tags to FLAC files
- Update Go Metadata struct with new fields
- Add GetDeezerExtendedMetadata export function for Flutter
- Register platform channel handlers for Android and iOS
- Pass genre/label through download flow to all services (Tidal/Qobuz/Amazon)

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

@@ -284,6 +284,13 @@ class MainActivity: FlutterActivity() {
                             }
                             result.success(response)
                         }
+                        "getDeezerExtendedMetadata" -> {
+                            val trackId = call.argument<String>("track_id") ?: ""
+                            val response = withContext(Dispatchers.IO) {
+                                Gobackend.getDeezerExtendedMetadata(trackId)
+                            }
+                            result.success(response)
+                        }
                         "convertSpotifyToDeezer" -> {
                             val resourceType = call.argument<String>("resource_type") ?: ""
                             val spotifyId = call.argument<String>("spotify_id") ?: ""
@@ -438,6 +445,14 @@ class MainActivity: FlutterActivity() {
                             }
                             result.success(null)
                         }
+                        "invokeExtensionAction" -> {
+                            val extensionId = call.argument<String>("extension_id") ?: ""
+                            val actionName = call.argument<String>("action") ?: ""
+                            val response = withContext(Dispatchers.IO) {
+                                Gobackend.invokeExtensionActionJSON(extensionId, actionName)
+                            }
+                            result.success(response)
+                        }
                         "searchTracksWithExtensions" -> {
                             val query = call.argument<String>("query") ?: ""
                             val limit = call.argument<Int>("limit") ?: 20