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/datasources/local_data_provider.dart

@@ -0,0 +1,38 @@
+import 'dart:convert';
+
+import 'package:injectable/injectable.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import '../../../common/constant/local_storage_key.dart';
+import '../unit_dtos.dart';
+
+/// Satuan berjumlah sedikit dan jarang berubah, jadi cukup disimpan sebagai
+/// satu blob JSON di SharedPreferences — tidak perlu tabel sqflite sendiri.
+@injectable
+class UnitLocalDataProvider {
+  final SharedPreferences _sharedPreferences;
+
+  UnitLocalDataProvider(this._sharedPreferences);
+
+  Future<void> saveUnits(List<UnitDto> units) async {
+    final jsonString = jsonEncode(units.map((e) => e.toJson()).toList());
+    await _sharedPreferences.setString(LocalStorageKey.units, jsonString);
+  }
+
+  List<UnitDto> getUnits() {
+    final jsonString = _sharedPreferences.getString(LocalStorageKey.units);
+    if (jsonString == null || jsonString.isEmpty) return const [];
+
+    final decoded = jsonDecode(jsonString);
+    if (decoded is! List) return const [];
+
+    return decoded
+        .whereType<Map<String, dynamic>>()
+        .map(UnitDto.fromJson)
+        .toList();
+  }
+
+  Future<void> clearUnits() async {
+    await _sharedPreferences.remove(LocalStorageKey.units);
+  }
+}

lib/infrastructure/unit/datasources/remote_data_provider.dart

@@ -0,0 +1,37 @@
+import 'dart:developer';
+
+import 'package:data_channel/data_channel.dart';
+import 'package:injectable/injectable.dart';
+
+import '../../../common/api/api_client.dart';
+import '../../../common/api/api_failure.dart';
+import '../../../common/function/app_function.dart';
+import '../../../common/url/api_path.dart';
+import '../../../domain/unit/unit.dart';
+import '../unit_dtos.dart';
+
+@injectable
+class UnitRemoteDataProvider {
+  final ApiClient _apiClient;
+  final _logName = 'UnitRemoteDataProvider';
+
+  UnitRemoteDataProvider(this._apiClient);
+
+  Future<DC<UnitFailure, List<UnitDto>>> fetchUnits() async {
+    try {
+      final response = await _apiClient.get(
+        ApiPath.units,
+        headers: getAuthorizationHeader(),
+      );
+
+      if (response.data['data'] == null) {
+        return DC.error(const UnitFailure.empty());
+      }
+
+      return DC.data(UnitDto.listFromResponse(response.data['data']));
+    } on ApiFailure catch (e, s) {
+      log('fetchUnitsError', name: _logName, error: e, stackTrace: s);
+      return DC.error(UnitFailure.serverError(e));
+    }
+  }
+}