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/auth/login_form/login_form_bloc.dart

@@ -6,6 +6,7 @@ import 'package:freezed_annotation/freezed_annotation.dart';
 import 'package:injectable/injectable.dart';
 
 import '../../../common/service/device_info_service.dart';
+import '../../../common/service/unit_registry.dart';
 import '../../../common/service/fcm_service.dart';
 import '../../../domain/auth/auth.dart';
 
@@ -18,11 +19,13 @@ class LoginFormBloc extends Bloc<LoginFormEvent, LoginFormState> {
   final IAuthRepository _authRepository;
   final FcmService _fcmService;
   final DeviceInfoService _deviceInfoService;
+  final UnitRegistry _unitRegistry;
 
   LoginFormBloc(
     this._authRepository,
     this._fcmService,
     this._deviceInfoService,
+    this._unitRegistry,
   ) : super(LoginFormState.initial()) {
     on<LoginFormEvent>(_onLoginFormEvent);
   }
@@ -62,6 +65,12 @@ class LoginFormBloc extends Bloc<LoginFormEvent, LoginFormState> {
             devicePayload: devicePayload,
           );
 
+          // Master satuan diambil sekali per sesi supaya keranjang bisa
+          // menulis "4,2 ons" tanpa menunggu request saat kasir menimbang.
+          if (failureOrLogin.isRight()) {
+            await _unitRegistry.load(forceRefresh: true);
+          }
+
           emit(
             state.copyWith(
               isSubmitting: false,