Can't get web auth to work with my own authentication server

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Hello,

I have my own authentication service to authenticate my users. The service is written in Go. The authentication flow works like this:

- the client queries `http://localhost:8080/auth/facebook`
- the client is redirected to a facebook page where the user will authorize my app
- facebook then redirects to `http://localhost:8080/auth/callback` with the user info (I do stuff with this data, like adding the user to a database)

The above works. I'm now trying to build the UI. So far I have something like this:

```dart
import 'package:flutter/material.dart';
import 'package:social_login_buttons/social_login_buttons.dart';
import 'package:flutter_web_auth/flutter_web_auth.dart';

void main() {
runApp(const MyApp());
}

class MyApp extends StatelessWidget {
const MyApp({super.key});

@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Sample',
home: LoginPage(),
);
}
}

class LoginPage extends StatelessWidget {
LoginPage({super.key});

@override
Widget build(BuildContext context) {

return LayoutBuilder(builder: (context, constraints) {
return Scaffold(
body: SocialLoginButton(
buttonType: SocialLoginButtonType.facebook,
onPressed: () async {
final result = await FlutterWebAuth.authenticate(
url: "http://localhost:8080/auth/facebook", callbackUrlScheme: "callback-scheme");

print("result: $result");

final token = await Uri.parse(result).queryParameters['token'];
print(token);
},
),
);
});
}
}
```

On the server side, this is the implementation of the `/auth/callback` endpoint:

```go
func AuthCallback(c *gin.Context) {
user, err := gothic.CompleteUserAuth(c.Writer, c.Request)

...

c.HTML(http.StatusOK, "auth.html", gin.H{})
// c.Redirect(http.StatusFound, "callback-scheme://")
}
```

I tried serving the `auth.html` page mentioned [here](https://github.com/LinusU/flutter_web_auth#web) from the server. With this setup, I can click the Facebook button, the web page opens, I can login on Facebook, and I can give permissions to the app. I'm then redirected to the `auth.html` page, and then...nada. I never get back anything for `final result =...` (on the client side). There is no error in the flutter console.

I tried other stuff on the server side:

```go
c.JSON(http.StatusOK, gin.H{"token": "hello"})
```
With this, I ultimately a json blob with the `hello` token in the web page that opens, but I'm not getting anything back on the client side.

I'm missing something, but I don't know what. Would you be able to help please?

Start with the FlutterWebAuth.authenticate call and the server-side AuthCallback endpoint, then inspect the auth.html page referenced in the issue. Trace the redirect from the completed Facebook flow and determine what response the client receives; done means final result is populated with the callback data or token.

Written by the indexing model from the issue text.