feat: skeletons where the page waits, no flicker while a sheet is read, repeated copies in the preview

Reading a large sheet repainted the list and "Sua folha" on every progress
step, so the preview flickered; progress now moves the bar in place and
"Sua folha" shows a skeleton until the preview is ready. A sheet printed
N times is shown N times (up to six, then a count). The payment page shows
skeletons while the order loads and until the card form is ready.

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

web/site-upload.js

@@ -84,7 +84,7 @@ function sel(fs){
           let mostrado=0;
           previaPngGrande(x.f, 600, p=>{
             const pct=60+Math.floor(p*38);
-            if(pct-mostrado>=3){ mostrado=pct; x.pct=pct; pintaFolha(); }
+            if(pct-mostrado>=1){ mostrado=pct; x.pct=pct; pintaProgressoFolha(x); }
           }).then(cv=>{
             if(cv && folhas.includes(x)){
               x.previewSrc=cv.toDataURL('image/webp',0.85);