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.

ios/Runner/AppDelegate.swift

@@ -393,6 +393,12 @@ import Gobackend  // Import Go framework
             let insecureTLS = args["insecure_tls"] as? Bool ?? false
             GobackendSetNetworkCompatibilityOptions(allowHTTP, insecureTLS)
             return nil
+
+        case "setAllowPrivateNetwork":
+            let args = call.arguments as! [String: Any]
+            let allowed = args["allowed"] as? Bool ?? false
+            GobackendSetAllowPrivateNetwork(allowed)
+            return nil
             
         case "checkDuplicate":
             let args = call.arguments as! [String: Any]