feat(01-11): implement auth-aware router and splash screen

- Created AppRouter with GoRouter for declarative navigation
- Added protected routes that redirect to login if not authenticated
- Implemented splash screen with loading state and auth checking
- Set up route definitions for login, signup, home, and splash
- Added error handling for navigation failures
- Includes authentication state-based redirects

Files modified:
- lib/core/router/app_router.dart
- lib/features/authentication/presentation/pages/splash_page.dart

lib/core/router/app_router.dart

@@ -0,0 +1,159 @@
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:supabase_flutter/supabase_flutter.dart';
+
+import '../../providers/auth_provider.dart';
+import '../../features/authentication/presentation/pages/login_page.dart';
+import '../../features/authentication/presentation/pages/signup_page.dart';
+import '../../features/home/presentation/pages/home_page.dart';
+import '../../features/authentication/presentation/pages/splash_page.dart';
+
+/// Application router configuration
+/// 
+/// Handles navigation with authentication state awareness and protected routes
+class AppRouter {
+  static final GoRouter _router = GoRouter(
+    initialLocation: '/',
+    debugLogDiagnostics: true,
+    redirect: (context, state) {
+      final authState = state.extra as AuthState?;
+      
+      // If no auth state in extra, check from provider
+      if (authState == null) {
+        final container = ProviderScope.containerOf(context, listen: false);
+        final authNotifier = container.read(authStateProvider.notifier);
+        
+        // For now, we'll use Supabase directly for auth state checking
+        // This will be improved when auth provider is fully integrated
+        final currentUser = Supabase.instance.client.auth.currentUser;
+        
+        // Allow splash page regardless of auth state
+        if (state.location == '/splash') {
+          return null;
+        }
+        
+        // If not authenticated and trying to access protected route, redirect to login
+        if (currentUser == null && !state.location.startsWith('/login') && !state.location.startsWith('/signup')) {
+          return '/login';
+        }
+        
+        // If authenticated and on auth pages, redirect to home
+        if (currentUser != null && (state.location.startsWith('/login') || state.location.startsWith('/signup'))) {
+          return '/home';
+        }
+        
+        return null;
+      }
+      
+      // TODO: Update to use AuthState when fully integrated
+      // For now, use Supabase auth state directly
+      final currentUser = Supabase.instance.client.auth.currentUser;
+      
+      if (currentUser == null && !state.location.startsWith('/login') && !state.location.startsWith('/signup')) {
+        return '/login';
+      }
+      
+      if (currentUser != null && (state.location.startsWith('/login') || state.location.startsWith('/signup'))) {
+        return '/home';
+      }
+      
+      return null;
+    },
+    routes: [
+      // Splash route - initial loading screen
+      GoRoute(
+        path: '/splash',
+        name: 'splash',
+        builder: (context, state) => const SplashPage(),
+      ),
+      
+      // Authentication routes (public)
+      GoRoute(
+        path: '/login',
+        name: 'login',
+        builder: (context, state) => const LoginPage(),
+      ),
+      
+      GoRoute(
+        path: '/signup',
+        name: 'signup',
+        builder: (context, state) => const SignupPage(),
+      ),
+      
+      // Protected routes (require authentication)
+      GoRoute(
+        path: '/home',
+        name: 'home',
+        builder: (context, state) => const HomePage(),
+      ),
+      
+      // Root route - redirects based on auth state
+      GoRoute(
+        path: '/',
+        redirect: (context, state) {
+          final currentUser = Supabase.instance.client.auth.currentUser;
+          return currentUser != null ? '/home' : '/splash';
+        },
+      ),
+      
+      // Additional routes will be added here
+      // Example:
+      // GoRoute(
+      //   path: '/inventory',
+      //   name: 'inventory',
+      //   builder: (context, state) => const InventoryPage(),
+      // ),
+    ],
+    errorBuilder: (context, state) => Scaffold(
+      appBar: AppBar(
+        title: const Text('Error'),
+      ),
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            const Icon(
+              Icons.error_outline,
+              size: 64,
+              color: Colors.red,
+            ),
+            const SizedBox(height: 16),
+            Text(
+              'Page not found',
+              style: Theme.of(context).textTheme.headlineSmall,
+            ),
+            const SizedBox(height: 8),
+            Text(
+              'Could not find: ${state.location}',
+              style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+                color: Colors.grey[600],
+              ),
+            ),
+            const SizedBox(height: 24),
+            ElevatedButton(
+              onPressed: () {
+                // Navigate to home or login based on auth state
+                final currentUser = Supabase.instance.client.auth.currentUser;
+                if (currentUser != null) {
+                  context.go('/home');
+                } else {
+                  context.go('/login');
+                }
+              },
+              child: const Text('Go Home'),
+            ),
+          ],
+        ),
+      ),
+    ),
+  );
+
+  /// Get the router instance
+  static GoRouter get router => _router;
+}
+
+/// Router provider for Riverpod integration
+final routerProvider = Provider<GoRouter>((ref) {
+  return AppRouter.router;
+});

lib/features/authentication/presentation/pages/splash_page.dart

@@ -0,0 +1,140 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:supabase_flutter/supabase_flutter.dart';
+
+/// Splash page for initial loading and authentication state checking
+/// 
+/// Shows loading indicator while checking authentication state
+/// and redirects to appropriate screen based on auth status
+class SplashPage extends ConsumerStatefulWidget {
+  const SplashPage({super.key});
+
+  @override
+  ConsumerState<SplashPage> createState() => _SplashPageState();
+}
+
+class _SplashPageState extends ConsumerState<SplashPage> {
+  @override
+  void initState() {
+    super.initState();
+    _checkAuthState();
+  }
+
+  /// Check authentication state and navigate accordingly
+  Future<void> _checkAuthState() async {
+    try {
+      // Add small delay for better UX - shows splash screen
+      await Future.delayed(const Duration(milliseconds: 1500));
+      
+      if (!mounted) return;
+      
+      // Check current authentication state
+      final currentUser = Supabase.instance.client.auth.currentUser;
+      
+      if (currentUser != null) {
+        // User is authenticated, navigate to home
+        // TODO: Use GoRouter when integrated
+        // Navigator.of(context).pushReplacementNamed('/home');
+        
+        // For now, we'll use a simple navigation
+        Navigator.of(context).pushReplacementNamed('/home');
+      } else {
+        // User is not authenticated, navigate to login
+        // TODO: Use GoRouter when integrated
+        // Navigator.of(context).pushReplacementNamed('/login');
+        
+        // For now, we'll use a simple navigation
+        Navigator.of(context).pushReplacementNamed('/login');
+      }
+    } catch (e) {
+      // Handle authentication state check errors
+      debugPrint('Error checking auth state: $e');
+      
+      if (!mounted) return;
+      
+      // On error, navigate to login for safety
+      Navigator.of(context).pushReplacementNamed('/login');
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      backgroundColor: Theme.of(context).colorScheme.primaryContainer,
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            // App logo/icon
+            Container(
+              width: 120,
+              height: 120,
+              decoration: BoxDecoration(
+                color: Theme.of(context).colorScheme.primary,
+                borderRadius: BorderRadius.circular(24),
+                boxShadow: [
+                  BoxShadow(
+                    color: Theme.of(context).colorScheme.primary.withOpacity(0.3),
+                    blurRadius: 20,
+                    offset: const Offset(0, 8),
+                  ),
+                ],
+              ),
+              child: const Icon(
+                Icons.inventory_2_outlined,
+                size: 60,
+                color: Colors.white,
+              ),
+            ),
+            
+            const SizedBox(height: 32),
+            
+            // App name
+            Text(
+              'Sage',
+              style: Theme.of(context).textTheme.headlineMedium?.copyWith(
+                fontWeight: FontWeight.bold,
+                color: Theme.of(context).colorScheme.primary,
+              ),
+            ),
+            
+            const SizedBox(height: 8),
+            
+            // Tagline
+            Text(
+              'Your Food Inventory Tracker',
+              style: Theme.of(context).textTheme.bodyLarge?.copyWith(
+                color: Theme.of(context).colorScheme.onSurface.withOpacity(0.7),
+              ),
+            ),
+            
+            const SizedBox(height: 48),
+            
+            // Loading indicator
+            Column(
+              children: [
+                SizedBox(
+                  width: 24,
+                  height: 24,
+                  child: CircularProgressIndicator(
+                    strokeWidth: 2,
+                    valueColor: AlwaysStoppedAnimation<Color>(
+                      Theme.of(context).colorScheme.primary,
+                    ),
+                  ),
+                ),
+                const SizedBox(height: 16),
+                Text(
+                  'Loading...',
+                  style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+                    color: Theme.of(context).colorScheme.onSurface.withOpacity(0.6),
+                  ),
+                ),
+              ],
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}