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>

tests/browser_test.mjs

@@ -99,6 +99,10 @@ try{
   try{
     // A cart the Site can price is approved at once: the customer pays now.
     await waitFor(async()=> (await site.text()).includes('Total validado no servidor:'),'browser upload and automatic approval',45000);
+    // Payment is its own page: the cart form is gone and the order summary is shown.
+    assert.equal(await site.eval('location.pathname'),'/pagamento');
+    assert.equal(await site.eval('getComputedStyle(document.getElementById("carr")).display'),'none');
+    assert.equal(await site.eval('!document.getElementById("pagResumo").hidden && document.getElementById("pagResumo").textContent.includes("Total")'),true);
   }catch(error){
     console.error('Checkout status:',await site.eval('document.getElementById("checkoutStatus")?.textContent'));
     throw error;