fix: stop Site timers from running after a product is closed

The grade check and the layout preview run on short timers. When the
item went to the cart inside that window, no product was open and both
threw in the customer's browser, which also failed the browser tests
intermittently. Each now returns when no product is open. The cart test
waits for the empty state, which is painted on the next animation frame.

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

tests/browser_test.mjs

@@ -87,6 +87,8 @@ try{
   await waitFor(()=>site.eval(`location.pathname==='/carrinho' && getComputedStyle($('carr')).display!=='none' && getComputedStyle($('foco')).display==='none'`),'cart page');
   // Removing an item is one click and can be undone.
   await site.click('[data-rmi="0"]');
+  // The empty state is painted on the next animation frame (site-steps.js).
+  await waitFor(()=>site.eval(`!$('carrVazio').hidden||document.documentElement.hasAttribute('data-sem-itens')`),'empty cart state');
   assert.deepEqual(await site.eval(`({items:pedido.length,empty:!$('carrVazio').hidden||document.documentElement.hasAttribute('data-sem-itens'),undo:!$('desfazer').hidden})`),{items:0,empty:true,undo:true});
   await site.click('#desfazerBtn');
   assert.equal(await site.eval('pedido.length===1 && pedido[0].total===21.89 && $("desfazer").hidden'),true);