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/core/ui/app_shell.dart

@@ -210,8 +210,8 @@ class AppShell extends ConsumerWidget {
                   break;
               }
             },
-            itemBuilder: (context) => const [
-              PopupMenuItem<String>(
+            itemBuilder: (context) => [
+              const PopupMenuItem<String>(
                 value: 'profile',
                 child: ListTile(
                   leading: Icon(Icons.person_outline),
@@ -220,7 +220,7 @@ class AppShell extends ConsumerWidget {
                 ),
               ),
               if (isAdmin)
-                PopupMenuItem<String>(
+                const PopupMenuItem<String>(
                   value: 'admin',
                   child: ListTile(
                     leading: Icon(Icons.admin_panel_settings_outlined),
@@ -228,8 +228,8 @@ class AppShell extends ConsumerWidget {
                     contentPadding: EdgeInsets.zero,
                   ),
                 ),
-              PopupMenuDivider(),
-              PopupMenuItem<String>(
+              const PopupMenuDivider(),
+              const PopupMenuItem<String>(
                 value: 'logout',
                 child: ListTile(
                   leading: Icon(Icons.logout),