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_request_dto.dart

@@ -75,6 +75,7 @@ class OrderItemRequestDto with _$OrderItemRequestDto {
     @JsonKey(name: "quantity") int? quantity,
     @JsonKey(name: "unit_price") int? unitPrice,
     @JsonKey(name: "notes") String? notes,
+    @JsonKey(name: "weight") double? weight,
   }) = _OrderItemRequestDto;
 
   factory OrderItemRequestDto.fromJson(Map<String, dynamic> json) =>
@@ -87,6 +88,7 @@ class OrderItemRequestDto with _$OrderItemRequestDto {
     quantity: quantity ?? 0,
     unitPrice: unitPrice ?? 0,
     notes: notes ?? '',
+    weight: weight,
   );
 
   factory OrderItemRequestDto.fromDomain(OrderItemRequest request) =>
@@ -96,6 +98,7 @@ class OrderItemRequestDto with _$OrderItemRequestDto {
         quantity: request.quantity,
         unitPrice: request.unitPrice,
         notes: request.notes,
+        weight: request.weight,
       );
 
   Map<String, dynamic> toRequest() {
@@ -110,6 +113,12 @@ class OrderItemRequestDto with _$OrderItemRequestDto {
       data["product_variant_id"] = productVariantId;
     }
 
+    // Produk satuan ditolak backend bila membawa berat, jadi field ini hanya
+    // ikut saat memang ada hasil timbangan.
+    if (weight != null) {
+      data["weight"] = weight;
+    }
+
     return data;
   }
 }