feat: add custom/not_specified shipping modes with cost input for ML publish

- build_item_payload supports shipping_cost for custom mode with costs array
- Add shipping mode selector: me2, custom, not_specified
- Show shipping cost input when custom is selected
- Backend passes shipping_cost through custom_data to payload builder

pos/static/js/inventory.js

@@ -993,9 +993,20 @@
         }
     });
 
+    window.onMeliShippingChange = function() {
+        var mode = document.getElementById('meliShippingMode').value;
+        var costField = document.getElementById('meliShippingCostField');
+        if (costField) costField.style.display = (mode === 'custom') ? 'block' : 'none';
+    };
+
     function _collectMeliCustomData() {
         var ids = Array.from(selectedItems);
         var customData = { titles: {}, prices: {}, stocks: {}, attributes: {} };
+        var mode = document.getElementById('meliShippingMode').value;
+        if (mode === 'custom') {
+            var costEl = document.getElementById('meliShippingCost');
+            if (costEl) customData.shipping_cost = parseFloat(costEl.value) || 0;
+        }
         ids.forEach(function(id) {
             var titleEl = document.getElementById('meliTitle-' + id);
             var priceEl = document.getElementById('meliPrice-' + id);