Add reusable two-factor verification for login and money operations (#32)

frontend/app/lib/auth/form.dart

@@ -9,6 +9,7 @@ import 'package:sxchnge_analytics/sxchnge_analytics.dart';
 import 'package:sxchnge_auth/sxchnge_auth.dart';
 
 import '../routing/routes.dart';
+import '../security/challenge.dart';
 import 'feedback.dart';
 import 'validation.dart';
 
@@ -96,11 +97,34 @@ class _AuthFormState extends State<AuthForm> {
       _confirmation.clear();
       context.goNamed(AppRoutes.verificationSent, extra: email);
     } else {
-      await context.read<LoginProvider>().login(
+      final login = context.read<LoginProvider>();
+      await login.login(
         email: _email.text,
         password: _password.text,
         locale: locale,
       );
+      if (!mounted || _signUp) return;
+      final challenge = login.state.result?.challenge;
+      if (challenge != null) {
+        _password.clear();
+        final grant = await verifyFactor(
+          context,
+          challenge: challenge,
+          service: context.read<TwoFactorService>(),
+          recorder: context.read<JourneyRecorder>(),
+          journey: JourneyKind.login,
+        );
+        if (!mounted || _signUp) return;
+        if (grant == null) {
+          login.reset();
+          context.read<JourneyRecorder>().abandon(JourneyExit.navigation);
+          return;
+        }
+        await login.complete(challenge, grant);
+        if (mounted && login.state.status == AuthStatus.success) {
+          context.go('/accounts');
+        }
+      }
     }
   }