feat(boxing): support editing assigned box records

lib/pages/boxing_detail_page.dart

@@ -61,14 +61,37 @@ class BoxingDetailPage extends StatelessWidget {
   }
 
   Widget _buildBoxGroup(BuildContext context, BoxDetailData box) {
+    final boxTotalQuantity = box.items.fold<int>(
+      0,
+      (sum, item) => sum + item.quantity,
+    );
+
     return Padding(
       padding: const EdgeInsets.only(bottom: 12),
       child: Column(
         crossAxisAlignment: CrossAxisAlignment.start,
         children: [
-          Text(
-            '箱号 ${box.boxNo}',
-            style: const TextStyle(fontSize: 14, fontWeight: FontWeight.bold),
+          Row(
+            children: [
+              Expanded(
+                child: Text(
+                  '箱号 ${box.boxNo}',
+                  style: const TextStyle(
+                    fontSize: 14,
+                    fontWeight: FontWeight.bold,
+                  ),
+                ),
+              ),
+              Text(
+                '共 $boxTotalQuantity 件',
+                textAlign: TextAlign.right,
+                style: TextStyle(
+                  fontSize: 14,
+                  fontWeight: FontWeight.w600,
+                  color: Theme.of(context).colorScheme.primary,
+                ),
+              ),
+            ],
           ),
           const SizedBox(height: 4),
           Container(