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/application/order/order_form/order_form_bloc.dart

@@ -60,15 +60,7 @@ class OrderFormBloc extends Bloc<OrderFormEvent, OrderFormState> {
           notes: '',
           customerName: state.customerName ?? "",
           orderItems: e.items
-              .map(
-                (item) => OrderItemRequest(
-                  productId: item.product.id,
-                  productVariantId: item.variant?.id ?? "",
-                  quantity: item.quantity,
-                  unitPrice: item.product.price.toInt(),
-                  notes: item.notes,
-                ),
-              )
+              .map(OrderItemRequest.fromProductQuantity)
               .toList(),
         );
 
@@ -100,15 +92,7 @@ class OrderFormBloc extends Bloc<OrderFormEvent, OrderFormState> {
           notes: '',
           customerName: state.customerName ?? "",
           orderItems: e.items
-              .map(
-                (item) => OrderItemRequest(
-                  productId: item.product.id,
-                  productVariantId: item.variant?.id ?? "",
-                  quantity: item.quantity,
-                  unitPrice: item.product.price.toInt(),
-                  notes: item.notes,
-                ),
-              )
+              .map(OrderItemRequest.fromProductQuantity)
               .toList(),
         );
 
@@ -137,15 +121,7 @@ class OrderFormBloc extends Bloc<OrderFormEvent, OrderFormState> {
         );
 
         final request = e.items
-            .map(
-              (item) => AddItemOrderRequest(
-                productId: item.product.id,
-                productVariantId: item.variant?.id ?? '',
-                quantity: item.quantity,
-                unitPrice: item.product.price.toInt(),
-                notes: item.notes,
-              ),
-            )
+            .map(AddItemOrderRequest.fromProductQuantity)
             .toList();
 
         failureOrAddItemOrder = await _repository.addItemOrder(