feat: tell the customer how long the upload takes before it starts

When files are picked, the product page shows the range the upload takes
between a slow (10 Mbps) and a fast (150 Mbps) connection, and asks to keep
the page open. The cart shows the same range until the real speed is
measured, then the measured time left.

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

web/site-pdf.js

@@ -406,6 +406,7 @@ function detalhesDaFolha(a,i,aberto){
 }
 function pintaFolha(){
   invalidaItemAtual();
+  pintaAvisoEnvio();
   const L=larguraFilme();
   pintaTipoEnvio();                        // trava o seletor enquanto houver folha
   $('zona').classList.toggle('compacta', folhas.length>0);
@@ -528,6 +529,7 @@ function pintaProgressoArte(a){
 }
 function pintaArtes(){
   invalidaItemAtual();
+  pintaAvisoEnvio();
   $('zona').classList.toggle('compacta', artes.length>0);
   // The same card as a finished sheet: thumbnail, size, a DPI chip, then the
   // width (with the usual sizes), copies, rotation and mirroring.