feat: rebuild the Site product page around a buy box

Artworks on the left; on the right a box that stays in view with the live
sheet, the grade, the price per metre, the metres charged, the total, the
resolution note and "Adicionar ao carrinho". The separate quality and
preview panels, the second sheet preview, the per-row mini sheets, the
summary box and the repeated findings list are gone: each piece of
information now appears once.

Each artwork row carries at most one hint (resolution first, otherwise a
width that saves film), the ready-sheet/loose-artwork choice is a toggle
beside the title, the upload area is one bar and the tips are collapsed.
The box only shows the item the page already priced, so it always matches
the cart.

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

web/site-steps.js

@@ -71,11 +71,10 @@ function vaiPara(url){
     const r=el.getBoundingClientRect();
     return r.height>0 && r.top<innerHeight && r.bottom>0;
   }
-  // Where the phone bar's button leads: the button that finishes the item on a
-  // product page, the order summary in the cart.
+  // Where the phone bar's button leads: the add-to-cart button on a product
+  // page, the order summary in the cart.
   function alvoBarra(){
-    if(rota()==='produto' && itemAtual)
-      return $('prev').classList.contains('on') ? $('pOk') : $('qOk');
+    if(rota()==='produto' && itemAtual) return $('bAdd');
     if(rota()==='carrinho' && (pedido.length || itemAtual) && !temCheckout())
       return document.querySelector('.resumo');
     return null;