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/domain/order/entities/order_entity.dart

@@ -197,19 +197,19 @@ class Order with _$Order {
   );
 
   // For Add Item Order
-  factory Order.fromLastOrder(Order order, List<ProductQuantity> products) =>
-      order.copyWith(
-        orderItems: products
-            .map((e) => OrderItem.fromProductQuantity(e))
-            .toList(),
-        totalAmount: products
-            .map((e) => e.product.price.toInt() * e.quantity)
-            .reduce((value, element) => value + element),
-            subtotal: products
-            .map((e) => e.product.price.toInt() * e.quantity)
-            .reduce((value, element) => value + element),
-            metadata: order.metadata,
-      );
+  factory Order.fromLastOrder(Order order, List<ProductQuantity> products) {
+    final subtotal = products.fold<int>(
+      0,
+      (sum, item) => sum + item.lineTotal.toInt(),
+    );
+
+    return order.copyWith(
+      orderItems: products.map(OrderItem.fromProductQuantity).toList(),
+      totalAmount: subtotal,
+      subtotal: subtotal,
+      metadata: order.metadata,
+    );
+  }
 
   List<OrderItem> get orderItemNotCancelled =>
       orderItems.where((item) => item.status != 'cancelled').toList();
@@ -217,6 +217,8 @@ class Order with _$Order {
 
 @freezed
 class OrderItem with _$OrderItem {
+  const OrderItem._();
+
   const factory OrderItem({
     required String id,
     required String orderId,
@@ -237,6 +239,12 @@ class OrderItem with _$OrderItem {
     required int paidQuantity,
     required String categoryId,
     required String categoryName,
+
+    /// Angka timbangan baris ini. `null` untuk produk cacah.
+    double? weight,
+    @Default('') String unitId,
+    @Default('') String unitName,
+    @Default('') String unitAbbreviation,
   }) = _OrderItem;
 
   factory OrderItem.empty() => OrderItem(
@@ -269,10 +277,9 @@ class OrderItem with _$OrderItem {
         productName: productQuantity.product.name,
         productVariantId: productQuantity.variant?.id ?? '',
         productVariantName: productQuantity.variant?.name ?? '',
-        quantity: productQuantity.quantity,
+        quantity: productQuantity.isWeighted ? 1 : productQuantity.quantity,
         unitPrice: productQuantity.product.price.toInt(),
-        totalPrice: (productQuantity.product.price * productQuantity.quantity)
-            .toInt(),
+        totalPrice: productQuantity.lineTotal.toInt(),
         modifiers: [],
         notes: productQuantity.notes,
         status: 'pending',
@@ -283,5 +290,30 @@ class OrderItem with _$OrderItem {
         paidQuantity: 0,
         categoryId: '',
         categoryName: '',
+        weight: productQuantity.weight,
+        unitId: productQuantity.product.unitId,
+        unitName: productQuantity.product.unitName,
+        unitAbbreviation: productQuantity.product.unitAbbreviation,
       );
+
+  /// Baris hasil penimbangan — utuh, tidak bisa dipecah sebagian.
+  bool get isWeighted => weight != null && weight! > 0;
+
+  /// Singkatan satuan untuk ditampilkan; jatuh ke nama panjang bila kosong.
+  String get unitLabel =>
+      unitAbbreviation.isNotEmpty ? unitAbbreviation : unitName;
+
+  /// `4,2 ons` untuk baris berbobot, `2` untuk baris biasa.
+  ///
+  /// Jangan menampilkan [quantity] mentah pada baris berbobot — nilainya selalu
+  /// 1 dan terbaca seperti "satu ons".
+  String get quantityLabel {
+    if (!isWeighted) return quantity.toString();
+    final value = weight!.weightFormat;
+    return unitLabel.isEmpty ? value : '$value $unitLabel';
+  }
+
+  /// `4,2 ons x Rp 4.500` atau `2 x Rp 25.000`.
+  String get quantityPriceLabel =>
+      '$quantityLabel x ${unitPrice.currencyFormatRpV2}';
 }