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/index.html

@@ -917,6 +917,7 @@ footer a:hover{color:var(--laranja2)}
             <div class="trocouCam" id="avisoTipo" style="display:none"></div>
             <div class="recusa" id="recusa" style="display:none"></div>
             <div id="lista"></div>
+            <p class="avisoEnvio" id="avisoEnvio" role="note" hidden></p>
             <div class="zona" id="zona" tabindex="0" role="button">
               <div class="ico">↑</div>
               <b id="zTit">Arraste aqui</b><span id="zSub"></span>