fix: ffi instance be singleton

ui/flutter/lib/api/native/channel/libgopeed_channel.dart

@@ -6,17 +6,15 @@ class LibgopeedChannel implements LibgopeedAbi {
   static const _channel = MethodChannel('gopeed.com/libgopeed');
 
   @override
-  Future<String> create(String cfg) async {
-    final result = await _channel.invokeMethod<String>('create', {
+  Future<void> init(String cfg) async {
+    await _channel.invokeMethod<String>('init', {
       'cfg': cfg,
     });
-    return result!;
   }
 
   @override
-  Future<String> invoke(int instance, String params) async {
+  Future<String> invoke(String params) async {
     final result = await _channel.invokeMethod<String>('invoke', {
-      'instance': instance,
       'params': params,
     });
     return result!;