feat: the preview of a large file says how long is left, not a percentage

Reading a large PNG in the browser for its preview showed "gerando a
prévia… 87%". It now shows the time left, worked out from how long the
reading has taken so far: "faltam cerca de 25 s", "faltam cerca de 3 min",
"quase pronto". The bar still fills as it goes.

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

web/site-upload.js

@@ -65,10 +65,11 @@ function sel(fs){
           x.an={dpi:md.dpiFolha, grande:true, artes:0, residuos:0, encostadas:0,
                 fundoChapado:false, aproveitamento:0, alturaCm:md.alt, fonteDpi:'arquivo'};
           if(!/\.png$/i.test(x.f.name)){ x.pct=null; pintaFolha(); return; }
-          x.pct=60; pintaFolha();
+          x.pct=60; x.leitura={inicio:Date.now(), p:0}; pintaFolha();
           let mostrado=0;
           previaPngGrande(x.f, 600, p=>{
             const pct=60+Math.floor(p*38);
+            x.leitura.p=p;
             if(pct-mostrado>=1){ mostrado=pct; x.pct=pct; pintaProgressoFolha(x); }
           }).then(cv=>{
             if(cv && folhas.includes(x)){
@@ -131,10 +132,11 @@ function medir(a){
       // JPG and WebP cannot be read in parts: the art is placed by its
       // measurements, as a full box, and the team checks it before printing.
       if(!/\.png$/i.test(a.f.name)){ a.semPrevia=true; return a; }
-      a.lendo=0; pintaArtes();
+      a.lendo=0; a.leitura={inicio:Date.now(), p:0}; pintaArtes();
       let mostrado=0;
       const cv=await previaPngGrande(a.f, 1200, p=>{
         const pct=Math.floor(p*100);
+        a.leitura.p=p;
         if(pct-mostrado>=1){ mostrado=pct; a.lendo=pct; pintaProgressoArte(a); }
       }).catch(()=>null);
       a.lendo=null;