Add auth session flow and sign-in UX

test/widget_test.dart

@@ -1,5 +1,7 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:relationship_saver/core/auth/in_memory_token_store.dart';
+import 'package:relationship_saver/integrations/backend/backend_gateway_provider.dart';
 import 'package:relationship_saver/main.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
@@ -8,11 +10,16 @@ void main() {
     SharedPreferences.setMockInitialValues(<String, Object>{});
   });
 
-  testWidgets('renders dashboard flow', (WidgetTester tester) async {
-    await tester.pumpWidget(const ProviderScope(child: RelationshipSaverApp()));
+  testWidgets('renders sign-in flow by default', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      ProviderScope(
+        overrides: [tokenStoreProvider.overrideWithValue(InMemoryTokenStore())],
+        child: const RelationshipSaverApp(),
+      ),
+    );
     await tester.pumpAndSettle();
 
-    expect(find.text('Today'), findsOneWidget);
-    expect(find.text('Active People'), findsOneWidget);
+    expect(find.text('Sign in to continue'), findsOneWidget);
+    expect(find.text('Sign In'), findsOneWidget);
   });
 }