Add auth session flow and sign-in UX

lib/main.dart

@@ -1,22 +1,36 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:relationship_saver/core/config/app_theme.dart';
+import 'package:relationship_saver/features/auth/session_controller.dart';
+import 'package:relationship_saver/features/auth/sign_in_view.dart';
 import 'package:relationship_saver/features/home/app_shell.dart';
+import 'package:relationship_saver/integrations/backend/models/backend_models.dart';
 
 void main() {
   runApp(const ProviderScope(child: RelationshipSaverApp()));
 }
 
-class RelationshipSaverApp extends StatelessWidget {
+class RelationshipSaverApp extends ConsumerWidget {
   const RelationshipSaverApp({super.key});
 
   @override
-  Widget build(BuildContext context) {
+  Widget build(BuildContext context, WidgetRef ref) {
+    final AsyncValue<AuthSession?> sessionState = ref.watch(
+      sessionControllerProvider,
+    );
+
     return MaterialApp(
       title: 'Relationship Saver',
       debugShowCheckedModeBanner: false,
       theme: AppTheme.light(),
-      home: const AppShell(),
+      home: Scaffold(
+        body: sessionState.when(
+          data: (session) =>
+              session == null ? const SignInView() : const AppShell(),
+          loading: () => const Center(child: CircularProgressIndicator()),
+          error: (Object error, StackTrace stackTrace) => const SignInView(),
+        ),
+      ),
     );
   }
 }