feat: show upload progress with a bar, time left and a header badge

Uploads of several GB ran behind one grey line in the cart. The order
summary now has a progress bar with the percentage, size and an estimate of
the time left from the recent speed; the header shows "enviando 45%" on
every page while it runs; the payment button says what it is waiting for;
and leaving the page mid-upload asks first.

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

web/index.html

@@ -765,7 +765,7 @@ footer a:hover{color:var(--laranja2)}
   </div>
   <div class="acts">
     <a href="/portal.html">Minha conta</a>
-    <a href="/carrinho" id="cartLink">Carrinho (0)</a>
+    <a href="/carrinho" id="cartLink">Carrinho (0)</a><span class="cartEnvio" id="cartEnvio" aria-live="polite"></span>
   </div>
 </div></header>