feat: give payment its own page

The PIX and card choices appeared under the cart, on the same page as the
customer's details. "Ir para o pagamento" now sends the order and opens
/pagamento, step 3 of the progress bar: the server's order summary, then PIX
or card, each opening below. The cart keeps only the sending progress and its
errors; a changed cart is sent again instead of offering the old quote.
Portal links open the payment page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/index.html

@@ -869,7 +869,7 @@ footer a:hover{color:var(--laranja2)}
   <i aria-hidden="true"></i>
   <a href="/carrinho" data-passo="2"><span>2</span>Dados e entrega</a>
   <i aria-hidden="true"></i>
-  <a href="/carrinho#checkoutStatus" data-passo="3"><span>3</span>Pagamento</a>
+  <a href="/pagamento" data-passo="3"><span>3</span>Pagamento</a>
 </nav>
 <div class="w">
 
@@ -1209,6 +1209,11 @@ footer a:hover{color:var(--laranja2)}
   <!-- Fora de #carr de propósito: o pedido pago esvazia o carrinho, e a
        confirmação não pode sumir junto com o painel. -->
   <div class="checkout">
+    <div class="pagCab">
+      <h3>Pagamento</h3>
+      <a href="/carrinho" class="pagVolta">Voltar ao carrinho</a>
+    </div>
+    <div class="pagResumo" id="pagResumo" hidden></div>
     <p id="checkoutStatus" class="avisoE" role="status" aria-live="polite"></p>
     <div id="checkoutActions"></div>
   </div>