feat: add deeplinking support for oauth login

lib/features/onboarding/view/callback_page.dart

@@ -0,0 +1,121 @@
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+import 'package:hooks_riverpod/hooks_riverpod.dart';
+import 'package:vaani/features/onboarding/providers/oauth_provider.dart';
+import 'package:vaani/features/onboarding/view/user_login_with_password.dart';
+import 'package:vaani/models/error_response.dart';
+import 'package:vaani/router/router.dart';
+
+class CallbackPage extends HookConsumerWidget {
+  CallbackPage({super.key, this.state, this.code});
+
+  final String? state;
+  final String? code;
+
+  final serverErrorResponse = ErrorResponseHandler();
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    if (state == null || code == null) {
+      return _SomethingWentWrong(
+        message:
+            'OAuth callback missing state or code\nGot: State: $state, Code: $code',
+      );
+    }
+
+    final flows = ref.read(oauthFlowsProvider);
+
+    // check if the state is in the flows
+    if (!flows.containsKey(state)) {
+      return const _SomethingWentWrong(
+        message: 'State not found',
+      );
+    }
+
+    // get the token
+    final loginAuthToken = ref.watch(
+      loginInExchangeForCodeProvider(
+        oauthState: state!,
+        code: code!,
+        responseHandler: serverErrorResponse,
+      ),
+    );
+    return Scaffold(
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            Text('Contacting server...\nPlease wait\n\nGot:'
+                '\nState: $state\nCode: $code'),
+            loginAuthToken.when(
+              data: (authenticationToken) {
+                if (authenticationToken == null) {
+                  handleServerError(
+                    context,
+                    serverErrorResponse,
+                  );
+                  return const BackToLoginButton();
+                }
+                return Text('Token: $authenticationToken');
+              },
+              loading: () => const CircularProgressIndicator(),
+              error: (error, _) {
+                handleServerError(
+                  context,
+                  serverErrorResponse,
+                  e: error,
+                );
+                return Column(
+                  children: [
+                    Text('Error with OAuth flow: $error'),
+                    const BackToLoginButton(),
+                  ],
+                );
+              },
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+class BackToLoginButton extends StatelessWidget {
+  const BackToLoginButton({
+    super.key,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return ElevatedButton(
+      onPressed: () {
+        GoRouter.of(context).goNamed(Routes.onboarding.name);
+      },
+      child: const Text('Return to login'),
+    );
+  }
+}
+
+class _SomethingWentWrong extends StatelessWidget {
+  const _SomethingWentWrong({
+    super.key,
+    this.message = 'Error with OAuth flow',
+  });
+
+  final String message;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            Text(message),
+            const BackToLoginButton(),
+          ],
+        ),
+      ),
+    );
+  }
+}