Make sorting of invoice items reliable.

resources/views/invoice/create.blade.php

@@ -655,6 +655,7 @@
                 for (let i = 0; i < self.items.length; i++) {
                     let pos = sort_flipped[i];
                     let item = self.items[pos];
+                    item.sort = i;
                     const result = axios.post('invoice/' + self.invoice_id + '/item', item)
                         .catch(function (error) {
                             self.error = true;