Refactor code structure for improved readability and maintainability

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

flutter/lib/core/ui/app_shell.dart

@@ -4,6 +4,13 @@ import 'package:go_router/go_router.dart';
 
 import '../../features/auth/data/auth_providers.dart';
 
+const _adminDestination = _AppDestination(
+  path: '/admin',
+  title: 'Admin',
+  icon: Icons.admin_panel_settings_outlined,
+  label: 'Admin',
+);
+
 class AppShell extends ConsumerWidget {
   final String location;
   final Widget child;
@@ -14,7 +21,7 @@ class AppShell extends ConsumerWidget {
     required this.child,
   });
 
-  static const _destinations = [
+  static const _baseDestinations = [
     _AppDestination(
       path: '/recipes',
       title: 'Recept',
@@ -53,8 +60,13 @@ class AppShell extends ConsumerWidget {
     ),
   ];
 
-  int _selectedIndex() {
-    final index = _destinations.indexWhere(
+  List<_AppDestination> _destinations(bool isAdmin) => [
+        ..._baseDestinations,
+        if (isAdmin) _adminDestination,
+      ];
+
+  int _selectedIndex(List<_AppDestination> destinations) {
+    final index = destinations.indexWhere(
       (destination) => location.startsWith(destination.path),
     );
     return index < 0 ? 0 : index;
@@ -62,8 +74,10 @@ class AppShell extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
-    final selectedIndex = _selectedIndex();
-    final selectedDestination = _destinations[selectedIndex];
+    final isAdmin = ref.watch(isAdminProvider);
+    final dests = _destinations(isAdmin);
+    final selectedIndex = _selectedIndex(dests);
+    final selectedDestination = dests[selectedIndex];
     final isWide = MediaQuery.of(context).size.width >= 900;
 
     Future<void> logout() async {
@@ -74,7 +88,7 @@ class AppShell extends ConsumerWidget {
     }
 
     void navigateTo(int index) {
-      final target = _destinations[index].path;
+      final target = dests[index].path;
       if (target != location && context.mounted) {
         context.go(target);
       }
@@ -98,7 +112,7 @@ class AppShell extends ConsumerWidget {
                   selectedIndex: selectedIndex,
                   onDestinationSelected: navigateTo,
                   labelType: NavigationRailLabelType.all,
-                  destinations: _destinations
+                  destinations: dests
                       .map(
                         (destination) => NavigationRailDestination(
                           icon: Icon(destination.icon),
@@ -117,7 +131,7 @@ class AppShell extends ConsumerWidget {
           : NavigationBar(
               selectedIndex: selectedIndex,
               onDestinationSelected: navigateTo,
-              destinations: _destinations
+              destinations: dests
                   .map(
                     (destination) => NavigationDestination(
                       icon: Icon(destination.icon),