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/dtos/unit_dto.dart

@@ -0,0 +1,59 @@
+part of '../unit_dtos.dart';
+
+@freezed
+class UnitDto with _$UnitDto {
+  const UnitDto._();
+
+  const factory UnitDto({
+    @JsonKey(name: "id") String? id,
+    @JsonKey(name: "organization_id") String? organizationId,
+    @JsonKey(name: "name") String? name,
+    @JsonKey(name: "abbreviation") String? abbreviation,
+    @JsonKey(name: "is_active") bool? isActive,
+  }) = _UnitDto;
+
+  factory UnitDto.fromJson(Map<String, dynamic> json) =>
+      _$UnitDtoFromJson(json);
+
+  ProductUnit toDomain() => ProductUnit(
+    id: id ?? '',
+    organizationId: organizationId ?? '',
+    name: name ?? '',
+    abbreviation: abbreviation ?? '',
+    isActive: isActive ?? true,
+  );
+
+  factory UnitDto.fromDomain(ProductUnit unit) => UnitDto(
+    id: unit.id,
+    organizationId: unit.organizationId,
+    name: unit.name,
+    abbreviation: unit.abbreviation,
+    isActive: unit.isActive,
+  );
+
+  /// Response `/units` belum tentu berbentuk sama di setiap versi backend —
+  /// bisa `data: [...]` atau `data: { units: [...] }`. Terima keduanya supaya
+  /// POS tidak kehilangan satuan hanya karena pembungkusnya berbeda.
+  static List<UnitDto> listFromResponse(dynamic data) {
+    if (data is List) {
+      return data
+          .whereType<Map<String, dynamic>>()
+          .map(UnitDto.fromJson)
+          .toList();
+    }
+
+    if (data is Map<String, dynamic>) {
+      for (final key in const ['units', 'items', 'data']) {
+        final value = data[key];
+        if (value is List) {
+          return value
+              .whereType<Map<String, dynamic>>()
+              .map(UnitDto.fromJson)
+              .toList();
+        }
+      }
+    }
+
+    return const [];
+  }
+}