feat: choose which cart items to pay, with a thumbnail of each

Each cart item has a checkbox and a thumbnail of its first file (or its
extension when there is none). Only checked items are priced, quoted,
uploaded and paid; an unchecked item stays in the cart, dimmed, for a later
order, and stays there after the others are paid. The header count is the
whole cart; the summary counts what is being paid.

Also removes the "A conta nasce aqui…" and "Carrinho salvo neste
navegador…" texts.

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

web/index.html

@@ -1094,8 +1094,6 @@ footer a:hover{color:var(--laranja2)}
 
       <div class="cxE">
         <h4>Seus dados</h4>
-        <p class="subC">A conta nasce aqui, no pagamento. O CNPJ é o mesmo da nota fiscal,
-          e é por ele que suas artes ficam guardadas para o próximo pedido.</p>
         <div class="campos">
           <div class="cp"><label>CNPJ</label>
             <input id="fCnpj" autocomplete="off" inputmode="numeric" maxlength="18" placeholder="00.000.000/0000-00">