[FIX] pos_gift_card: use get_unit_price fn to get rounded value

Occasionally a rounding error can occur that returns trailing zeros and a integer on the end.
Since the values are compared, we could be comparing 2 values with a difference of 0.00000001 and end up with an error.

closes odoo/odoo#108613

Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>

addons/pos_gift_card/static/src/js/PaymentScreen.js

@@ -38,7 +38,7 @@ odoo.define('pos_gift_card.PaymentScreen', function(require) {
                                     fields: ['balance'],
                                   });
 
-                                if(Math.abs(line.price) > gift_card[0].balance) {
+                                if(Math.abs(line.get_unit_price()) > gift_card[0].balance) {
                                     await this.showPopup('ErrorPopup', {
                                         'title': _t("Gift Card Error"),
                                         'body': _t("Gift card balance is too low."),