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/main/pages/home/widgets/home_right_panel.dart

@@ -7,6 +7,7 @@ import 'package:flutter_bloc/flutter_bloc.dart';
 import '../../../../../../application/checkout/checkout_form/checkout_form_bloc.dart';
 import '../../../../../../common/extension/extension.dart';
 import '../../../../../../common/theme/theme.dart';
+import '../../../../../../domain/product/product.dart';
 import '../../../../../components/button/button.dart';
 import '../../../../../components/card/order_menu.dart';
 import '../../../../../components/spaces/space.dart';
@@ -21,15 +22,7 @@ class HomeRightPanel extends StatelessWidget {
   Widget build(BuildContext context) {
     return BlocBuilder<CheckoutFormBloc, CheckoutFormState>(
       builder: (context, state) {
-        final price = state.items.isEmpty
-            ? 0.0
-            : state.items
-                  .map(
-                    (e) =>
-                        (e.product.price + (e.variant?.priceModifier ?? 0)) *
-                        e.quantity,
-                  )
-                  .reduce((value, element) => value + element);
+        final price = state.items.cartTotal;
         log('🛒 Total Price: $price');
         return Align(
           alignment: Alignment.topCenter,
@@ -87,8 +80,12 @@ class HomeRightPanel extends StatelessWidget {
                       : ListView.separated(
                           shrinkWrap: true,
                           padding: const EdgeInsets.symmetric(horizontal: 16),
-                          itemBuilder: (context, index) =>
-                              OrderMenu(data: state.items[index]),
+                          itemBuilder: (context, index) => OrderMenu(
+                            // Baris berbobot bisa dihapus satu per satu, jadi
+                            // state tiap baris harus terikat ke identitasnya.
+                            key: ValueKey(state.items[index].lineId),
+                            data: state.items[index],
+                          ),
                           separatorBuilder: (context, index) =>
                               const SpaceHeight(1.0),
                           itemCount: state.items.length,