feat: implement core Point of Sale (POS) page and functionality.

Author: huyhandsomeee

frontend/src/pages/Pos/pos.jsx

@@ -386,7 +386,7 @@ export default function POS() {
   };
 
   const getItemStock = (item) => {
-    return getPositiveNumber(item?.stockQuantity ?? item?.stock);
+    return Math.max(0, getPositiveNumber(item?.stockQuantity ?? item?.stock));
   };
 
   const getComboRetailTotal = (combo) => {