feat: Add promo codes

templates/cart.html

@@ -77,6 +77,7 @@
                         <div class="col-md-12 col-lg-4">
                             <div class="bg-body-tertiary summary">
                                 <h3>Summary</h3>
+                                <form style="margin-bottom: 25px;" method="post" action="/cart/promo"><label class="form-label" for="promo">Promo Code</label><input class="form-control" type="text" placeholder="Promo Code" name="promo" value="{{promo}}"><span>{{promo_message}}</span></form>
                                 <h4 class="border-primary-subtle" style="border-color: var(--bs-primary-border-subtle);"><span class="text">Total USD</span><span class="price">US${{total_usd}}</span></h4>
                                 <h4><span class="text">Total</span><span class="price">{{total_hns}} HNS</span></h4><a class="btn btn-primary btn-lg d-block w-100" role="button" href="/payment">Checkout</a>
                             </div>