Refactor category handling in inventory and pantry forms

- Introduced `_categorySearchController` and `_categoryOptions` in both `_InventoryFormDialogState` and `_PantryFormDialogState` to manage category selection more effectively.
- Implemented `_flattenCategoryOptions` method to create a flat list of category options from nested category nodes.
- Updated the initialization and disposal of the new controllers to ensure proper resource management.
- Made minor adjustments to the PopupMenuItem definitions in `AppShell` for consistency.

flutter/lib/features/admin/presentation/admin_pantry_panel.dart

@@ -506,6 +506,9 @@ class _PantryFormDialog extends StatefulWidget {
 
 class _PantryFormDialogState extends State<_PantryFormDialog> {
   late final TextEditingController _locationController;
+  late final TextEditingController _categorySearchController;
+  late List<CategorySelectOption> _categoryOptions;
+  
   int? _ownerUserId;
   int? _productId;
   int? _categoryId;
@@ -522,11 +525,14 @@ class _PantryFormDialogState extends State<_PantryFormDialog> {
     _categoryId = initialProduct?.categoryId;
     _categoryPath = initialProduct?.categoryPath;
     _locationController = TextEditingController(text: initial?.location ?? '');
+    _categorySearchController = TextEditingController(text: _categoryPath ?? '');
+    _categoryOptions = _flattenCategoryOptions(widget.categories);
   }
 
   @override
   void dispose() {
     _locationController.dispose();
+    _categorySearchController.dispose();
     super.dispose();
   }
 
@@ -563,6 +569,20 @@ class _PantryFormDialogState extends State<_PantryFormDialog> {
         .toList();
   }
 
+  List<CategorySelectOption> _flattenCategoryOptions(
+    List<AdminCategoryNode> nodes, [
+    List<String> parents = const [],
+  ]) {
+    final result = <CategorySelectOption>[];
+    for (final node in nodes) {
+      final pathParts = [...parents, node.name];
+      final path = pathParts.join(' > ');
+      result.add((value: node.id.toString(), label: path));
+      result.addAll(_flattenCategoryOptions(node.children, pathParts));
+    }
+    return result;
+  }
+
   Future<void> _pickCategory() async {
     final selected = await CategoryThenProductPicker.showCategorySheet(
       context,