feat(extension): add custom URL handler support for extensions

- Add URLHandlerConfig to extension manifest (Go)
- Add HandleURL method to extension providers (Go)
- Add export functions for URL handling (Go)
- Add URLHandler class to extension_provider.dart (Flutter)
- Add platform bridge methods for URL handling (Flutter)
- Update fetchFromUrl to check extension URL handlers first
- Add Android/iOS native handlers for extension URL routing
- Update CHANGELOG with new feature

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

@@ -551,6 +551,27 @@ class MainActivity: FlutterActivity() {
                             }
                             result.success(response)
                         }
+                        // Extension URL Handler API
+                        "handleURLWithExtension" -> {
+                            val url = call.argument<String>("url") ?: ""
+                            val response = withContext(Dispatchers.IO) {
+                                Gobackend.handleURLWithExtensionJSON(url)
+                            }
+                            result.success(response)
+                        }
+                        "findURLHandler" -> {
+                            val url = call.argument<String>("url") ?: ""
+                            val response = withContext(Dispatchers.IO) {
+                                Gobackend.findURLHandlerJSON(url)
+                            }
+                            result.success(response)
+                        }
+                        "getURLHandlers" -> {
+                            val response = withContext(Dispatchers.IO) {
+                                Gobackend.getURLHandlersJSON()
+                            }
+                            result.success(response)
+                        }
                         // Extension Post-Processing API
                         "runPostProcessing" -> {
                             val filePath = call.argument<String>("file_path") ?: ""