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/presentation/pages/refund/pages/refund_success/widgets/refund_success_right_panel.dart

@@ -255,7 +255,7 @@ class RefundSuccessRightPanel extends StatelessWidget {
                   borderRadius: BorderRadius.circular(12),
                 ),
                 child: Text(
-                  '${item.quantity}x',
+                  (item.isWeighted ? item.quantityLabel : "${item.quantity}x"),
                   style: AppStyle.md.copyWith(
                     fontWeight: FontWeight.bold,
                     color: Colors.white,

lib/presentation/pages/refund/widgets/refund_left_panel.dart

@@ -62,7 +62,9 @@ class RefundLeftPanel extends StatelessWidget {
                         ),
                       ),
                     Text(
-                      'Qty: ${item.quantity} x Rp ${item.unitPrice.currencyFormatRpV2} ',
+                      item.isWeighted
+                          ? item.quantityPriceLabel
+                          : 'Qty: ${item.quantity} x ${item.unitPrice.currencyFormatRpV2}',
                       style: AppStyle.md.copyWith(
                         color: AppColor.textSecondary,
                       ),