Scaffold backend gateway and integration docs

test/widget_test.dart

@@ -0,0 +1,14 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:relationship_saver/main.dart';
+
+void main() {
+  testWidgets('renders app shell', (WidgetTester tester) async {
+    await tester.pumpWidget(const RelationshipSaverApp());
+
+    expect(find.text('Relationship Saver'), findsOneWidget);
+    expect(
+      find.textContaining('Backend gateway scaffold is ready.'),
+      findsOneWidget,
+    );
+  });
+}