fix: use dialog builder context for Navigator.pop in dialogs/sheets

Passing the outer widget context to Navigator.pop() inside a dialog or
bottom sheet builder caused GoRouter to pop a page route instead of the
modal, triggering the "no pages left to show" assertion.

Affects _showChangeDialog and _confirmGenerate in menu_screen.dart,
and _showAddMenu bottom sheet in products_screen.dart.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/features/products/products_screen.dart

@@ -8,7 +8,7 @@ import 'product_provider.dart';
 void _showAddMenu(BuildContext context) {
   showModalBottomSheet(
     context: context,
-    builder: (_) => SafeArea(
+    builder: (ctx) => SafeArea(
       child: Column(
         mainAxisSize: MainAxisSize.min,
         children: [
@@ -16,7 +16,7 @@ void _showAddMenu(BuildContext context) {
             leading: const Icon(Icons.edit_outlined),
             title: const Text('Добавить вручную'),
             onTap: () {
-              Navigator.pop(context);
+              Navigator.pop(ctx);
               context.push('/products/add');
             },
           ),
@@ -24,7 +24,7 @@ void _showAddMenu(BuildContext context) {
             leading: const Icon(Icons.document_scanner_outlined),
             title: const Text('Сканировать чек или фото'),
             onTap: () {
-              Navigator.pop(context);
+              Navigator.pop(ctx);
               context.push('/scan');
             },
           ),