feat: add equality operators, make history screen paginated, random performance updates

lib/viewmodels/bar_screen_view_model.dart

@@ -88,9 +88,11 @@ class BarScreenViewModel extends ChangeNotifier {
       throw Exception('Product is out of stock.');
     }
 
-    await barTabService.addProductToTab(tabId: tab.id, product: product);
-
-    await inventory.decreaseStock(product.id, 1);
+    await barTabService.addProductToTab(
+      tabId: tab.id,
+      product: product,
+      stockAdjustment: () => inventory.decreaseStock(product.id, 1),
+    );
 
     await _reloadTabs();
   }
@@ -101,14 +103,15 @@ class BarScreenViewModel extends ChangeNotifier {
     await barTabService.updateTabItemQuantity(
       tabItemId: item.id,
       quantity: quantity,
+      stockAdjustment: () async {
+        if (difference > 0) {
+          await inventory.decreaseStock(item.productId, difference);
+        } else if (difference < 0) {
+          await inventory.increaseStock(item.productId, -difference);
+        }
+      },
     );
 
-    if (difference > 0) {
-      await inventory.decreaseStock(item.productId, difference);
-    } else if (difference < 0) {
-      await inventory.increaseStock(item.productId, -difference);
-    }
-
     await _reloadTabs();
   }