feat(network): add opt-in allow local/private network setting

Add a setting that relaxes the SSRF guard so extensions and built-in network code can reach private/local/loopback targets, for users routing traffic through a local proxy or custom DNS. Disabled by default. Wired end-to-end: Go backend (SetAllowPrivateNetwork toggles isPrivateIP guard), Android/iOS platform bridge, Dart settings model/provider, and a toggle in Download settings.

lib/services/platform_bridge.dart

@@ -546,6 +546,12 @@ class PlatformBridge {
     });
   }
 
+  static Future<void> setAllowPrivateNetwork(bool allowed) async {
+    await _channel.invokeMethod('setAllowPrivateNetwork', {
+      'allowed': allowed,
+    });
+  }
+
   static Future<Map<String, dynamic>> checkDuplicate(
     String outputDir,
     String isrc,