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/void/void_form/void_form_bloc.dart

@@ -55,7 +55,10 @@ class VoidFormBloc extends Bloc<VoidFormEvent, VoidFormState> {
             (item) => item.id == entry.key,
           );
           if (item == null) continue;
-          newTotal += item.unitPrice.toInt() * entry.value;
+          // Baris berbobot di-void utuh; rupiahnya sudah weight x harga.
+          newTotal += item.isWeighted
+              ? item.totalPrice
+              : item.unitPrice.toInt() * entry.value;
         }
 
         emit(
@@ -92,7 +95,13 @@ class VoidFormBloc extends Bloc<VoidFormEvent, VoidFormState> {
                 productVariantName: originalItem.productVariantName,
                 quantity: voidQty,
                 unitPrice: originalItem.unitPrice,
-                totalPrice: (originalItem.unitPrice) * voidQty,
+                totalPrice: originalItem.isWeighted
+                    ? originalItem.totalPrice
+                    : (originalItem.unitPrice) * voidQty,
+                weight: originalItem.weight,
+                unitId: originalItem.unitId,
+                unitName: originalItem.unitName,
+                unitAbbreviation: originalItem.unitAbbreviation,
                 modifiers: originalItem.modifiers,
                 notes: originalItem.notes,
                 status: originalItem.status,