feat: implement user-scoped receipt aliases with global fallback; enhance alias management in admin panel

flutter/lib/core/api/api_paths.dart

@@ -10,6 +10,8 @@ class ProductApiPaths {
   static const aiCategorizeBulk = '/products/ai-categorize-bulk';
   static const deleted = '/products/deleted';
   static const merge = '/products/merge';
+  static String mergePreview(int sourceProductId, int targetProductId) =>
+      '/products/merge-preview?sourceProductId=$sourceProductId&targetProductId=$targetProductId';
   static String setStatus(int id) => '/products/$id/status';
   static String update(int id) => '/products/$id';
   static String remove(int id) => '/products/$id';
@@ -29,6 +31,11 @@ class ReceiptImportApiPaths {
   static const refreshCategories = '/receipt-import/refresh-categories';
 }
 
+class ReceiptAliasApiPaths {
+  static const list = '/receipt-aliases';
+  static String remove(int id) => '/receipt-aliases/$id';
+}
+
 class RecipeApiPaths {
   static const list = '/recipes';
   static String detail(int id) => '/recipes/$id';

flutter/lib/core/ui/app_shell.dart

@@ -62,7 +62,9 @@ class AppShell extends ConsumerWidget {
     ),
   ];
 
-  List<_AppDestination> _destinations(bool isAdmin) => _baseDestinations;
+  List<_AppDestination> _destinations(bool isAdmin) => isAdmin
+      ? [..._baseDestinations, _adminHeaderDestination]
+      : _baseDestinations;
 
   int? _selectedIndex(List<_AppDestination> destinations) {
     final index = destinations.indexWhere(