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/application/sync/sync_setting/sync_setting_bloc.dart

@@ -3,6 +3,7 @@ import 'package:dartz/dartz.dart';
 import 'package:freezed_annotation/freezed_annotation.dart';
 import 'package:injectable/injectable.dart';
 
+import '../../../common/service/unit_registry.dart';
 import '../../../domain/category/category.dart';
 import '../../../domain/product/product.dart';
 
@@ -14,9 +15,13 @@ part 'sync_setting_bloc.freezed.dart';
 class SyncSettingBloc extends Bloc<SyncSettingEvent, SyncSettingState> {
   final IProductRepository _productRepository;
   final ICategoryRepository _categoryRepository;
+  final UnitRegistry _unitRegistry;
 
-  SyncSettingBloc(this._productRepository, this._categoryRepository)
-    : super(SyncSettingState.initial()) {
+  SyncSettingBloc(
+    this._productRepository,
+    this._categoryRepository,
+    this._unitRegistry,
+  ) : super(SyncSettingState.initial()) {
     on<SyncSettingEvent>(_onSyncSettingEvent);
   }
 
@@ -97,6 +102,10 @@ class SyncSettingBloc extends Bloc<SyncSettingEvent, SyncSettingState> {
       ),
     );
 
+    // Satuan lebih dulu: produk timbangan butuh singkatannya untuk bisa
+    // ditulis "4,2 ons" begitu selesai disinkronkan.
+    await _unitRegistry.load(forceRefresh: true);
+
     // Sync categories first
     final categoryResult = await _categoryRepository.syncAllCategories();
 
@@ -169,6 +178,8 @@ class SyncSettingBloc extends Bloc<SyncSettingEvent, SyncSettingState> {
   Future<void> _onSyncProducts(Emitter<SyncSettingState> emit) async {
     emit(state.copyWith(isLoading: true, failureOptionSyncProduct: none()));
 
+    await _unitRegistry.load(forceRefresh: true);
+
     final result = await _productRepository.syncAllProducts();
 
     result.fold(