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/unit/entities/unit_entity.dart

@@ -0,0 +1,23 @@
+part of '../unit.dart';
+
+/// Satuan timbangan milik organisasi — ons, kg, gram.
+///
+/// [abbreviation] yang dipakai POS untuk mencetak `4,2 ons` di struk.
+@freezed
+class ProductUnit with _$ProductUnit {
+  const factory ProductUnit({
+    required String id,
+    required String organizationId,
+    required String name,
+    required String abbreviation,
+    required bool isActive,
+  }) = _ProductUnit;
+
+  factory ProductUnit.empty() => const ProductUnit(
+    id: '',
+    organizationId: '',
+    name: '',
+    abbreviation: '',
+    isActive: false,
+  );
+}

lib/domain/unit/failures/unit_failure.dart

@@ -0,0 +1,12 @@
+part of '../unit.dart';
+
+@freezed
+sealed class UnitFailure with _$UnitFailure {
+  const factory UnitFailure.serverError(ApiFailure failure) = _ServerError;
+  const factory UnitFailure.unexpectedError() = _UnexpectedError;
+  const factory UnitFailure.empty() = _Empty;
+  const factory UnitFailure.localStorageError(String erroMessage) =
+      _LocalStorageError;
+  const factory UnitFailure.dynamicErrorMessage(String erroMessage) =
+      _DynamicErrorMessage;
+}

lib/domain/unit/repositories/i_unit_repository.dart

@@ -0,0 +1,12 @@
+part of '../unit.dart';
+
+abstract class IUnitRepository {
+  /// Ambil satuan dari server lalu simpan ke cache lokal.
+  /// Bila server tidak terjangkau, kembalikan isi cache supaya POS tetap bisa
+  /// mencetak `4,2 ons` saat offline.
+  Future<Either<UnitFailure, List<ProductUnit>>> getUnits({
+    bool forceRefresh = false,
+  });
+
+  Future<List<ProductUnit>> cachedUnits();
+}

lib/domain/unit/unit.dart

@@ -0,0 +1,10 @@
+import 'package:dartz/dartz.dart';
+import 'package:freezed_annotation/freezed_annotation.dart';
+
+import '../../common/api/api_failure.dart';
+
+part 'unit.freezed.dart';
+
+part 'entities/unit_entity.dart';
+part 'failures/unit_failure.dart';
+part 'repositories/i_unit_repository.dart';