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/infrastructure/order/dtos/order_dto.dart

@@ -118,6 +118,10 @@ class OrderItemDto with _$OrderItemDto {
     @JsonKey(name: "paid_quantity") int? paidQuantity,
     @JsonKey(name: "category_id") String? categoryId,
     @JsonKey(name: "category_name") String? categoryName,
+    @JsonKey(name: "weight") double? weight,
+    @JsonKey(name: "unit_id") String? unitId,
+    @JsonKey(name: "unit_name") String? unitName,
+    @JsonKey(name: "unit_abbreviation") String? unitAbbreviation,
   }) = _OrderItemDto;
 
   factory OrderItemDto.fromJson(Map<String, dynamic> json) =>
@@ -144,6 +148,10 @@ class OrderItemDto with _$OrderItemDto {
     paidQuantity: paidQuantity ?? 0,
     categoryId: categoryId ?? '',
     categoryName: categoryName ?? '',
+    weight: weight,
+    unitId: unitId ?? '',
+    unitName: unitName ?? '',
+    unitAbbreviation: unitAbbreviation ?? '',
   );
 
   factory OrderItemDto.fromDomain(OrderItem orderItem) => OrderItemDto(
@@ -166,5 +174,9 @@ class OrderItemDto with _$OrderItemDto {
     paidQuantity: orderItem.paidQuantity,
     categoryId: orderItem.categoryId,
     categoryName: orderItem.categoryName,
+    weight: orderItem.weight,
+    unitId: orderItem.unitId,
+    unitName: orderItem.unitName,
+    unitAbbreviation: orderItem.unitAbbreviation,
   );
 }