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/product/dtos/product_dto.dart

@@ -24,6 +24,25 @@ class ListProductDto with _$ListProductDto {
   );
 }
 
+/// Satuan yang menempel pada produk.
+///
+/// Backend hanya dijamin mengirim `unit_id`; sebagian response ikut menyertakan
+/// objek `unit` lengkap. Kalau ada, dipakai langsung supaya POS tidak perlu
+/// menunggu master satuan termuat.
+@freezed
+class ProductUnitDto with _$ProductUnitDto {
+  const ProductUnitDto._();
+
+  const factory ProductUnitDto({
+    @JsonKey(name: "id") String? id,
+    @JsonKey(name: "name") String? name,
+    @JsonKey(name: "abbreviation") String? abbreviation,
+  }) = _ProductUnitDto;
+
+  factory ProductUnitDto.fromJson(Map<String, dynamic> json) =>
+      _$ProductUnitDtoFromJson(json);
+}
+
 @freezed
 class ProductDto with _$ProductDto {
   const ProductDto._();
@@ -46,11 +65,28 @@ class ProductDto with _$ProductDto {
     @JsonKey(name: "created_at") String? createdAt,
     @JsonKey(name: "updated_at") String? updatedAt,
     @JsonKey(name: "variants") List<ProductVariantDto>? variants,
+    @JsonKey(name: "sell_by") String? sellBy,
+    @JsonKey(name: "unit_id") String? unitId,
+    @JsonKey(name: "unit_name") String? unitName,
+    @JsonKey(name: "unit_abbreviation") String? unitAbbreviation,
+    @JsonKey(name: "unit") ProductUnitDto? unit,
   }) = _ProductDto;
 
   factory ProductDto.fromJson(Map<String, dynamic> json) =>
       _$ProductDtoFromJson(json);
 
+  String get _resolvedUnitId =>
+      (unitId?.isNotEmpty == true ? unitId : unit?.id) ?? '';
+
+  String get _resolvedUnitName =>
+      (unitName?.isNotEmpty == true ? unitName : unit?.name) ?? '';
+
+  String get _resolvedUnitAbbreviation =>
+      (unitAbbreviation?.isNotEmpty == true
+          ? unitAbbreviation
+          : unit?.abbreviation) ??
+      '';
+
   /// Mapping ke domain
   Product toDomain() => Product(
     id: id ?? '',
@@ -70,6 +106,10 @@ class ProductDto with _$ProductDto {
     createdAt: createdAt ?? '',
     updatedAt: updatedAt ?? '',
     variants: variants?.map((v) => v.toDomain()).toList() ?? [],
+    sellBy: SellBy.normalize(sellBy),
+    unitId: _resolvedUnitId,
+    unitName: _resolvedUnitName,
+    unitAbbreviation: _resolvedUnitAbbreviation,
   );
 
   Map<String, dynamic> toMap() => {
@@ -89,6 +129,10 @@ class ProductDto with _$ProductDto {
     'is_active': isActive == true ? 1 : 0,
     'created_at': createdAt,
     'updated_at': updatedAt,
+    'sell_by': SellBy.normalize(sellBy),
+    'unit_id': _resolvedUnitId,
+    'unit_name': _resolvedUnitName,
+    'unit_abbreviation': _resolvedUnitAbbreviation,
   };
 
   factory ProductDto.fromMap(
@@ -117,6 +161,10 @@ class ProductDto with _$ProductDto {
     createdAt: map['created_at'] as String?,
     updatedAt: map['updated_at'] as String?,
     variants: variants,
+    sellBy: map['sell_by'] as String?,
+    unitId: map['unit_id'] as String?,
+    unitName: map['unit_name'] as String?,
+    unitAbbreviation: map['unit_abbreviation'] as String?,
   );
 
   factory ProductDto.fromDomain(Product product) => ProductDto(
@@ -139,6 +187,10 @@ class ProductDto with _$ProductDto {
     variants: product.variants
         .map((v) => ProductVariantDto.fromDomain(v))
         .toList(),
+    sellBy: product.sellBy,
+    unitId: product.unitId,
+    unitName: product.unitName,
+    unitAbbreviation: product.unitAbbreviation,
   );
 }