Refactor code structure for improved readability and maintainability

Co-authored-by: Copilot <copilot@github.com>

flutter/lib/core/router/app_router.dart

@@ -5,6 +5,7 @@ import 'package:go_router/go_router.dart';
 import '../ui/app_shell.dart';
 import '../ui/async_state_views.dart';
 import '../../features/auth/data/auth_providers.dart';
+import '../../core/auth/jwt_decoder.dart';
 import '../../features/auth/presentation/login_screen.dart';
 import '../../features/profile/presentation/profile_screen.dart';
 import '../../features/recipes/presentation/create_recipe_screen.dart';
@@ -20,6 +21,7 @@ import '../../features/inventory/presentation/consumption_history_screen.dart';
 import '../../features/meal_plan/presentation/meal_plan_screen.dart';
 import '../../features/pantry/presentation/pantry_screen.dart';
 import '../../features/import/presentation/import_screen.dart';
+import '../../features/admin/presentation/admin_screen.dart';
 
 final appRouterProvider = Provider<GoRouter>((ref) {
   final authState = ref.watch(authStateProvider);
@@ -28,7 +30,7 @@ final appRouterProvider = Provider<GoRouter>((ref) {
     initialLocation: '/',
     redirect: (context, state) {
       final isLoading = authState.isLoading;
-      final token = authState.valueOrNull;
+      final token = authState.maybeWhen(data: (t) => t, orElse: () => null);
       final isLoggedIn = token != null && token.isNotEmpty;
       final location = state.matchedLocation;
       final isSplash = location == '/';
@@ -194,6 +196,14 @@ final appRouterProvider = Provider<GoRouter>((ref) {
             path: '/profile',
             builder: (context, state) => const ProfileScreen(),
           ),
+          GoRoute(
+            path: '/admin',
+            redirect: (context, state) {
+              final token = ref.read(authStateProvider).maybeWhen(data: (t) => t, orElse: () => null);
+              return jwtIsAdmin(token) ? null : '/recipes';
+            },
+            builder: (context, state) => const AdminScreen(),
+          ),
         ],
       ),
     ],