feat: Refactor routing and navigation structure with StatefulShellRoute

- Introduced a new function `_shellBranchIndexForPath` to determine the index of the shell branch based on the path.
- Replaced `ShellRoute` with `StatefulShellRoute.indexedStack` for better state management during navigation.
- Updated `AppShell` to handle navigation path changes and integrate with the new routing structure.
- Organized routes into `StatefulShellBranch` for better modularity and clarity.
- Enhanced admin panel functionality with improved alias management and UI updates.
- Added new methods in `ReceiptImportSessionNotifier` for managing selected items and edits more efficiently.
- Improved UI components in receipt import and admin panels for better performance and user experience.
- Added PageStorageKeys to various ListViews to maintain scroll positions across navigation.
- Documented performance goals and profiling strategies in a new PERFORMANCE.md file.

flutter/lib/core/router/app_router.dart

@@ -24,6 +24,17 @@ import '../../features/pantry/presentation/pantry_screen.dart';
 import '../../features/import/presentation/import_screen.dart';
 import '../../features/admin/presentation/admin_screen.dart';
 
+int? _shellBranchIndexForPath(String path) {
+  if (path.startsWith('/recipes')) return 0;
+  if (path.startsWith('/inventory')) return 1;
+  if (path.startsWith('/matsedel')) return 2;
+  if (path.startsWith('/baslager')) return 3;
+  if (path.startsWith('/import')) return 4;
+  if (path.startsWith('/profile')) return 5;
+  if (path.startsWith('/admin')) return 6;
+  return null;
+}
+
 final appRouterProvider = Provider<GoRouter>((ref) {
   final authState = ref.watch(authStateProvider);
 
@@ -175,42 +186,90 @@ final appRouterProvider = Provider<GoRouter>((ref) {
         },
       ),
       // Shell routes — shared AppShell with navigation bar.
-      ShellRoute(
-        builder: (context, state, child) {
-          return AppShell(location: state.uri.path, child: child);
-        },
-        routes: [
-          GoRoute(
-            path: '/recipes',
-            builder: (context, state) => const RecipesScreen(),
-          ),
-          GoRoute(
-            path: '/inventory',
-            builder: (context, state) => const InventoryScreen(),
-          ),
-          GoRoute(
-            path: '/matsedel',
-            builder: (context, state) => const MealPlanScreen(),
-          ),
-          GoRoute(
-            path: '/baslager',
-            builder: (context, state) => const PantryScreen(),
-          ),
-          GoRoute(
-            path: '/import',
-            builder: (context, state) => const ImportScreen(),
-          ),
-          GoRoute(
-            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';
+      StatefulShellRoute.indexedStack(
+        builder: (context, state, navigationShell) {
+          return AppShell(
+            location: state.uri.path,
+            onNavigateToPath: (path) {
+              final index = _shellBranchIndexForPath(path);
+              if (index == null) {
+                context.go(path);
+                return;
+              }
+
+              if (index == navigationShell.currentIndex) {
+                if (state.uri.path != path) {
+                  context.go(path);
+                }
+                return;
+              }
+
+              navigationShell.goBranch(index);
             },
-            builder: (context, state) => const AdminScreen(),
+            child: navigationShell,
+          );
+        },
+        branches: [
+          StatefulShellBranch(
+            routes: [
+              GoRoute(
+                path: '/recipes',
+                builder: (context, state) => const RecipesScreen(),
+              ),
+            ],
+          ),
+          StatefulShellBranch(
+            routes: [
+              GoRoute(
+                path: '/inventory',
+                builder: (context, state) => const InventoryScreen(),
+              ),
+            ],
+          ),
+          StatefulShellBranch(
+            routes: [
+              GoRoute(
+                path: '/matsedel',
+                builder: (context, state) => const MealPlanScreen(),
+              ),
+            ],
+          ),
+          StatefulShellBranch(
+            routes: [
+              GoRoute(
+                path: '/baslager',
+                builder: (context, state) => const PantryScreen(),
+              ),
+            ],
+          ),
+          StatefulShellBranch(
+            routes: [
+              GoRoute(
+                path: '/import',
+                builder: (context, state) => const ImportScreen(),
+              ),
+            ],
+          ),
+          StatefulShellBranch(
+            routes: [
+              GoRoute(
+                path: '/profile',
+                builder: (context, state) => const ProfileScreen(),
+              ),
+            ],
+          ),
+          StatefulShellBranch(
+            routes: [
+              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(),
+              ),
+            ],
           ),
         ],
       ),