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/unit/unit_dtos.g.dart

@@ -0,0 +1,25 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'unit_dtos.dart';
+
+// **************************************************************************
+// JsonSerializableGenerator
+// **************************************************************************
+
+_$UnitDtoImpl _$$UnitDtoImplFromJson(Map<String, dynamic> json) =>
+    _$UnitDtoImpl(
+      id: json['id'] as String?,
+      organizationId: json['organization_id'] as String?,
+      name: json['name'] as String?,
+      abbreviation: json['abbreviation'] as String?,
+      isActive: json['is_active'] as bool?,
+    );
+
+Map<String, dynamic> _$$UnitDtoImplToJson(_$UnitDtoImpl instance) =>
+    <String, dynamic>{
+      'id': instance.id,
+      'organization_id': instance.organizationId,
+      'name': instance.name,
+      'abbreviation': instance.abbreviation,
+      'is_active': instance.isActive,
+    };