feat: make server owner of tabs

test/bar_screen_view_model_test.dart

@@ -4,7 +4,6 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:kooltap/models/bar_tab.dart';
 import 'package:kooltap/models/tab_item.dart';
 import 'package:kooltap/services/bar_tab_service.dart';
-import 'package:kooltap/services/product_service.dart';
 import 'package:kooltap/viewmodels/bar_screen_view_model.dart';
 import 'package:mocktail/mocktail.dart';
 
@@ -87,7 +86,7 @@ void main() {
     expect(viewModel.isLoading, false);
   });
 
-  test('propagates an insufficient-stock error from the transaction', () async {
+  test('propagates an error from the transaction', () async {
     const item = TabItem(
       id: 'item-1',
       tabId: 'tab-1',
@@ -99,11 +98,11 @@ void main() {
 
     when(
       () => barTabService.adjustTabItemQuantity(tabItemId: 'item-1', delta: 1),
-    ).thenAnswer((_) async => throw const InsufficientStockException());
+    ).thenAnswer((_) async => throw const TabNotFoundException('tab-1'));
 
     await expectLater(
       viewModel.changeItemQuantity(item, 1),
-      throwsA(isA<InsufficientStockException>()),
+      throwsA(isA<TabNotFoundException>()),
     );
 
     expect(viewModel.loadError, isNull);