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/auth_services_test.dart

@@ -0,0 +1,104 @@
+import 'dart:async';
+
+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';
+import 'package:sxchnge_app/providers/locale.dart';
+
+class ControlledLogin implements LoginService {
+  final calls = <Completer<AuthResult>>[];
+  LoginRequest? request;
+  @override
+  Future<AuthResult> login(LoginRequest input) {
+    request = input;
+    final completer = Completer<AuthResult>();
+    calls.add(completer);
+    return completer.future;
+  }
+}
+
+void main() {
+  testWidgets(
+    'app shows loading, localized failure, and retries through injected service',
+    (tester) async {
+      tester.view.physicalSize = const Size(1440, 1200);
+      tester.view.devicePixelRatio = 1;
+      addTearDown(tester.view.resetPhysicalSize);
+      addTearDown(tester.view.resetDevicePixelRatio);
+      final service = ControlledLogin();
+      await tester.pumpWidget(
+        SxchngeApp(
+          loginService: service,
+          localeProvider: LocaleProvider(initialLocale: const Locale('ru')),
+        ),
+      );
+      await tester.pumpAndSettle();
+      await tester.enterText(
+        find.byKey(const ValueKey('email')),
+        'a@example.com',
+      );
+      await tester.enterText(
+        find.byKey(const ValueKey('password')),
+        'example-password',
+      );
+      final button = find.byKey(const ValueKey('submit'));
+      await tester.ensureVisible(button);
+      await tester.tap(button);
+      await tester.pump();
+      expect(tester.widget<FilledButton>(button).onPressed, isNull);
+      expect(find.text('Подождите…'), findsOneWidget);
+      expect(service.request!.locale, 'ru');
+      service.calls.single.completeError(
+        const AuthException(AuthError.unavailable),
+      );
+      await tester.pumpAndSettle();
+      expect(
+        find.text('Сервис недоступен. Попробуйте ещё раз.'),
+        findsOneWidget,
+      );
+      await tester.tap(button);
+      await tester.pump();
+      expect(service.calls, hasLength(2));
+      service.calls.last.complete(
+        const AuthResult(userId: 'demo', isSimulated: true),
+      );
+      await tester.pumpAndSettle();
+      expect(find.byKey(const ValueKey('preview-result')), findsOneWidget);
+      expect(find.byKey(const ValueKey('auth-error')), findsNothing);
+      expect(tester.takeException(), isNull);
+    },
+  );
+
+  testWidgets('changing mode while loading ignores the old login result', (
+    tester,
+  ) async {
+    final service = ControlledLogin();
+    await tester.pumpWidget(SxchngeApp(loginService: service));
+    await tester.pumpAndSettle();
+    await tester.enterText(
+      find.byKey(const ValueKey('email')),
+      'a@example.com',
+    );
+    await tester.enterText(
+      find.byKey(const ValueKey('password')),
+      'example-password',
+    );
+    await tester.ensureVisible(find.byKey(const ValueKey('submit')));
+    await tester.tap(find.byKey(const ValueKey('submit')));
+    await tester.pump();
+    await tester.ensureVisible(find.text('Sign up'));
+    await tester.tap(find.text('Sign up'));
+    await tester.pumpAndSettle();
+    service.calls.single.complete(
+      const AuthResult(userId: 'demo', isSimulated: true),
+    );
+    await tester.pumpAndSettle();
+    expect(find.byKey(const ValueKey('confirmation')), findsOneWidget);
+    expect(find.byKey(const ValueKey('preview-result')), findsNothing);
+    expect(tester.takeException(), isNull);
+  });
+}