feat: pack large artworks in Artes avulsas from a streamed copy

Artes avulsas decoded every file whole, and the packing decoded each one
again on every repaint, so a large artwork failed to load. A PNG over 150
MB is now measured from its header (pixel size, so DPI and grade stay
exact) and packed from a 1200-pixel copy read as a stream, with progress on
its card; every artwork's image is loaded once and reused by the packing.
A large JPG or WebP asks for a PNG, and the grade card no longer calls a
file unreadable while it is still being read.

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

web/site-upload.js

@@ -136,6 +136,24 @@ function sel(fs){
   Promise.all(novos.map(medir)).then(pintaArtes);
 }
 function medir(a){
+  // A large artwork is measured from its header and packed from a small copy
+  // read as a stream (previaPngGrande); the file itself is never decoded.
+  if(a.f.size>GRANDE_BYTES){
+    return dimensoesImagem(a.f).then(async d=>{
+      if(!d || !(d.w>0) || !(d.h>0)){ a.decodeError=true; return a; }
+      a.px=d.w; a.py=d.h; a.prop=d.h/d.w; a.grande=true;
+      if(!/\.png$/i.test(a.f.name)){ a.decodeError='grande'; return a; }
+      a.lendo=0; pintaArtes();
+      let mostrado=0;
+      const cv=await previaPngGrande(a.f, 1200, p=>{
+        const pct=Math.floor(p*100);
+        if(pct-mostrado>=1){ mostrado=pct; a.lendo=pct; pintaProgressoArte(a); }
+      }).catch(()=>null);
+      a.lendo=null;
+      if(cv && artes.includes(a)) a.src=cv.toDataURL('image/webp',0.9); else if(!cv) a.decodeError=true;
+      return a;
+    }).catch(()=>{ a.decodeError=true; return a; });
+  }
   return carregarImagem(a.f).then(img=>{
     if(img){ a.prop=img.height/img.width; a.px=img.width; a.py=img.height; a.src=img.src; }
     else a.decodeError=true;