Deliver localized auth app with verification and go_router

Integrate themed login/signup, EN/RU localization, shared dummy providers, verification recovery screens and named URL routes. Store frontend guidelines, workflow skills, validation and delivery documentation. Related to #2, #3, #5 and #6.

frontend/app/test/verification_test.dart

@@ -0,0 +1,136 @@
+import 'package:flutter/material.dart';
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:sxchnge_auth/sxchnge_auth.dart';
+
+import 'package:sxchnge_app/main.dart';
+
+class RecoveringService implements VerificationService {
+  int sends = 0;
+  int confirmations = 0;
+  @override
+  Future<VerificationResult> send({
+    required String email,
+    required String locale,
+  }) async {
+    if (++sends == 1) throw StateError('private error');
+    return const VerificationResult(isSimulated: true);
+  }
+
+  @override
+  Future<VerificationResult> confirm({required String token}) async {
+    if (++confirmations == 1) throw StateError('private error');
+    return const VerificationResult(isSimulated: true);
+  }
+}
+
+Future<void> openVerification(
+  WidgetTester tester,
+  String route, {
+  VerificationService service = const DummyVerificationService(
+    delay: Duration.zero,
+  ),
+}) async {
+  tester.platformDispatcher.defaultRouteNameTestValue = route;
+  addTearDown(tester.platformDispatcher.clearDefaultRouteNameTestValue);
+  await tester.pumpWidget(SxchngeApp(verificationService: service));
+  await tester.pumpAndSettle();
+}
+
+void main() {
+  for (final scenario in {
+    'demo-valid': 'verified',
+    'demo-expired': 'expired',
+    'wrong': 'invalid',
+    '': 'invalid',
+    'demo-internal': 'error',
+  }.entries) {
+    testWidgets(
+      'confirmation link resolves ${scenario.key} as ${scenario.value}',
+      (tester) async {
+        await openVerification(tester, '/verify-email?token=${scenario.key}');
+        expect(
+          find.byKey(ValueKey('verification-${scenario.value}')),
+          findsOneWidget,
+        );
+        expect(find.textContaining('DEMO'), findsOneWidget);
+        expect(tester.takeException(), isNull);
+      },
+    );
+  }
+  testWidgets('internal confirmation error can retry without entering email', (
+    tester,
+  ) async {
+    final service = RecoveringService();
+    await openVerification(
+      tester,
+      '/verify-email?token=opaque',
+      service: service,
+    );
+    expect(find.text('private error'), findsNothing);
+    final retry = find.byKey(const ValueKey('verification-retry'));
+    await tester.ensureVisible(retry);
+    await tester.tap(retry);
+    await tester.pumpAndSettle();
+    expect(find.byKey(const ValueKey('verification-verified')), findsOneWidget);
+    expect(service.confirmations, 2);
+  });
+  testWidgets(
+    'request validates email, reports send error and resends successfully',
+    (tester) async {
+      final service = RecoveringService();
+      await openVerification(tester, '/verification-sent', service: service);
+      final resend = find.byKey(const ValueKey('verification-resend'));
+      await tester.ensureVisible(resend);
+      await tester.tap(resend);
+      await tester.pumpAndSettle();
+      expect(find.text('Enter your email address.'), findsOneWidget);
+      expect(service.sends, 0);
+      await tester.enterText(
+        find.byKey(const ValueKey('verification-email')),
+        'alex@example.com',
+      );
+      await tester.ensureVisible(resend);
+      await tester.tap(resend);
+      await tester.pumpAndSettle();
+      expect(find.byKey(const ValueKey('verification-error')), findsOneWidget);
+      await tester.ensureVisible(resend);
+      await tester.tap(resend);
+      await tester.pumpAndSettle();
+      expect(find.byKey(const ValueKey('verification-sent')), findsOneWidget);
+      expect(service.sends, 2);
+    },
+  );
+  for (final route in [
+    '/verification-sent',
+    '/verify-email?token=demo-valid',
+    '/verify-email?token=demo-expired',
+    '/verify-email?token=bad',
+    '/verify-email?token=demo-internal',
+  ]) {
+    testWidgets('$route fits narrow viewport at 200% in EN and RU', (
+      tester,
+    ) async {
+      tester.view.physicalSize = const Size(320, 740);
+      tester.view.devicePixelRatio = 1;
+      tester.platformDispatcher.textScaleFactorTestValue = 2;
+      addTearDown(tester.view.resetPhysicalSize);
+      addTearDown(tester.view.resetDevicePixelRatio);
+      addTearDown(tester.platformDispatcher.clearTextScaleFactorTestValue);
+      await openVerification(tester, route);
+      expect(tester.takeException(), isNull);
+      await tester.tap(find.byKey(const ValueKey('locale-selector')));
+      await tester.pumpAndSettle();
+      await tester.tap(
+        find.ancestor(
+          of: find.text('Русский'),
+          matching: find.byType(CheckedPopupMenuItem<Locale>),
+        ),
+      );
+      await tester.pumpAndSettle();
+      expect(find.text('RU'), findsOneWidget);
+      expect(tester.takeException(), isNull);
+    });
+  }
+}