feat: dukung penjualan produk per timbangan

Produk kini punya cara jual `unit` (cacah) atau `weight` (timbangan),
mengikuti kontrak backend di docs/integration-weight-based-products.md.

- tipe SellBy, domain/infrastructure unit baru, dan endpoint /api/v1/units
- dialog input berat saat produk timbangan ditambahkan ke keranjang
- tiap penimbangan jadi baris keranjang sendiri; total baris dihitung
  berat x harga satuan, sedangkan produk cacah tetap digabung
- request ke backend mengirim quantity 1 dan membawa berat
- migrasi DB v3: kolom sell_by, unit_id, unit_name, unit_abbreviation
  di tabel products supaya satuan tetap tampil saat offline
- tampilan keranjang, struk, split bill, void, dan refund memakai berat
- test/weight_based_products_test.dart menutup perilaku di atas (9 test)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

lib/presentation/pages/checkout/checkout_page.dart

@@ -7,6 +7,7 @@ import '../../../application/order/order_form/order_form_bloc.dart';
 import '../../../application/payment_method/payment_method_loader/payment_method_loader_bloc.dart';
 import '../../../application/table/table_loader/table_loader_bloc.dart';
 import '../../../common/theme/theme.dart';
+import '../../../domain/product/product.dart';
 import '../../../injection.dart';
 import '../../components/spaces/space.dart';
 import '../../components/toast/flushbar.dart';
@@ -86,14 +87,7 @@ class CheckoutPage extends StatelessWidget implements AutoRouteWrapper {
           tag: 'checkout_screen',
           child: BlocBuilder<CheckoutFormBloc, CheckoutFormState>(
             builder: (context, state) {
-              final price = state.items.fold(
-                0,
-                (previousValue, element) =>
-                    previousValue +
-                    (element.product.price.toInt() +
-                            (element.variant?.priceModifier.toInt() ?? 0)) *
-                        element.quantity,
-              );
+              final price = state.items.cartTotal.toInt();
 
               return Scaffold(
                 backgroundColor: AppColor.white,