fix(spa): use couponError to fix build warning

apps/spa/src/components/CartDrawer.tsx

@@ -207,6 +207,7 @@ function CartDrawerFooter({ subtotal, onClose, t }: { subtotal: number; onClose:
       
       {/* Coupon Input */}
       {!savedCoupon && (
+        <div className="flex flex-col gap-1">
           <div className="flex gap-2">
             <input
               type="text"
@@ -223,6 +224,8 @@ function CartDrawerFooter({ subtotal, onClose, t }: { subtotal: number; onClose:
               {couponLoading ? '...' : t('cart.applyCoupon', 'Apply')}
             </button>
           </div>
+          {couponError && <span className="text-xs text-rose-500 font-medium">{couponError}</span>}
+        </div>
       )}
 
       {/* Totals */}