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/components/card/product_card.dart

@@ -17,18 +17,35 @@ class ProductCard extends StatelessWidget {
   Widget build(BuildContext context) {
     return GestureDetector(
       onTap: () {
-        if (product.isActive == true) {
-          if (product.variants.isEmpty) {
-            context.read<CheckoutFormBloc>().add(
-              CheckoutFormEvent.addItem(product, null),
-            );
-          } else {
-            showDialog(
-              context: context,
-              builder: (context) => VariantDialog(product: product),
-            );
-          }
+        if (product.isActive != true) return;
+
+        if (product.variants.isNotEmpty) {
+          showDialog(
+            context: context,
+            builder: (context) => VariantDialog(product: product),
+          );
+          return;
         }
+
+        // Produk timbangan tidak bisa langsung masuk keranjang — kasir harus
+        // menimbang dulu, dan tiap penimbangan jadi barisnya sendiri.
+        if (product.isSoldByWeight) {
+          final bloc = context.read<CheckoutFormBloc>();
+          showDialog(
+            context: context,
+            builder: (context) => WeightInputDialog(
+              product: product,
+              onSubmit: (weight) => bloc.add(
+                CheckoutFormEvent.addWeightedItem(product, null, weight),
+              ),
+            ),
+          );
+          return;
+        }
+
+        context.read<CheckoutFormBloc>().add(
+          CheckoutFormEvent.addItem(product, null),
+        );
       },
       child: Container(
         decoration: BoxDecoration(
@@ -75,7 +92,11 @@ class ProductCard extends StatelessWidget {
                   Align(
                     alignment: Alignment.center,
                     child: Text(
-                      product.price.currencyFormatRp,
+                      product.isSoldByWeight
+                          ? '${product.price.currencyFormatRp} ${product.priceUnitLabel}'
+                              .trim()
+                          : product.price.currencyFormatRp,
+                      textAlign: TextAlign.center,
                       style: const TextStyle(
                         fontWeight: FontWeight.bold,
                         fontSize: 12,
@@ -89,31 +110,39 @@ class ProductCard extends StatelessWidget {
 
             BlocBuilder<CheckoutFormBloc, CheckoutFormState>(
               builder: (context, state) {
-                final totalQuantity = state.items
+                final lines = state.items
                     .where((item) => item.product.id == product.id)
-                    .map((item) => item.quantity)
-                    .fold(0, (sum, qty) => sum + qty);
+                    .toList();
+
+                if (lines.isEmpty) return const SizedBox.shrink();
+
+                // Untuk produk timbangan, jumlah baris tidak berarti apa-apa
+                // (selalu 1 per penimbangan) — yang informatif adalah total
+                // berat yang sudah masuk keranjang.
+                final label = product.isSoldByWeight
+                    ? '${lines.fold<double>(0, (sum, item) => sum + (item.weight ?? 0)).weightFormat} ${product.unitLabel}'
+                          .trim()
+                    : lines
+                          .fold<int>(0, (sum, item) => sum + item.quantity)
+                          .toString();
 
-                if (totalQuantity == 0) {
-                  return const SizedBox.shrink();
-                }
                 return Positioned(
                   top: 4,
                   right: 4,
                   child: Container(
-                    width: 40,
+                    constraints: const BoxConstraints(minWidth: 40),
                     height: 40,
-                    padding: const EdgeInsets.all(6),
+                    padding: const EdgeInsets.symmetric(horizontal: 6),
                     decoration: const BoxDecoration(
                       borderRadius: BorderRadius.all(Radius.circular(9.0)),
                       color: AppColor.primary,
                     ),
                     child: Center(
                       child: Text(
-                        totalQuantity.toString(),
-                        style: const TextStyle(
+                        label,
+                        style: TextStyle(
                           color: Colors.white,
-                          fontSize: 20,
+                          fontSize: product.isSoldByWeight ? 13 : 20,
                           fontWeight: FontWeight.bold,
                         ),
                       ),