Implement multi-screen app UI flow and navigation

lib/main.dart

@@ -1,7 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
-import 'package:relationship_saver/core/config/app_config.dart';
 import 'package:relationship_saver/core/config/app_theme.dart';
+import 'package:relationship_saver/features/home/app_shell.dart';
 
 void main() {
   runApp(const ProviderScope(child: RelationshipSaverApp()));
@@ -16,40 +16,7 @@ class RelationshipSaverApp extends StatelessWidget {
       title: 'Relationship Saver',
       debugShowCheckedModeBanner: false,
       theme: AppTheme.light(),
-      home: const _LandingScreen(),
-    );
-  }
-}
-
-class _LandingScreen extends StatelessWidget {
-  const _LandingScreen();
-
-  @override
-  Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(title: const Text('Relationship Saver')),
-      body: Padding(
-        padding: const EdgeInsets.all(24),
-        child: Column(
-          crossAxisAlignment: CrossAxisAlignment.start,
-          children: <Widget>[
-            Text(
-              'Backend gateway scaffold is ready.',
-              style: Theme.of(context).textTheme.headlineSmall,
-            ),
-            const SizedBox(height: 12),
-            Text(
-              'Mode: ${AppConfig.useFakeBackend ? 'Fake gateway' : 'REST gateway'}',
-              style: Theme.of(context).textTheme.bodyLarge,
-            ),
-            const SizedBox(height: 8),
-            Text(
-              'Base URL: ${AppConfig.backendBaseUrl}',
-              style: Theme.of(context).textTheme.bodyMedium,
-            ),
-          ],
-        ),
-      ),
+      home: const AppShell(),
     );
   }
 }