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>

deploy/nginx.conf.template

@@ -49,7 +49,7 @@ server {
     # Always revalidate HTML/JS/CSS after a deployment. Without this, a browser
     # can pair a new Kanban page with a cached older script after a rollout.
     # The Site's product pages and cart are addresses of the same page (web/site-pages.js).
-    location ~ ^/(arquivo-por-metro|artes-avulsas|uv-arquivo-por-metro|uv-artes-avulsas|carrinho)/?$ {
+    location ~ ^/(arquivo-por-metro|artes-avulsas|uv-arquivo-por-metro|uv-artes-avulsas|carrinho|pagamento)/?$ {
         expires -1;
         try_files /index.html =404;
     }