feat: redesign the Montagem page, with thumbnails, check chips and a DPI price scale

The product page is rebuilt around mockup 1. The two ways to send are
cards under the title; the PNG/CDR route choice is gone (it never changed
the price, and the picker now takes every allowed format). Each file card
has a thumbnail, its size, check chips with a plain sentence for any
problem, a "Ver detalhes da conferência" link and a copies stepper; a CDR
file says "Conferência manual · preço cheio" and asks for the length. The
buy box shows the price scale in DPI with the customer's step, the metre
and 10 cm length, the total and what the resolution saved, and "Sai em N
partes" only above 20 m. Folha já montada gets "Ampliar" instead of zoom.
Pricing, grade and checks are unchanged.

Fixes on top: the card thumbnail is a small image made once, not the full
preview re-parsed on every repaint; sizes from 1 GB up read in GB; a failed
upload is shown with "Tentar de novo" instead of retrying silently on every
cart change. The browser suites wait for the product and the reloaded page
instead of racing them, and the artwork suite delays imagemDaArte, which
the packing now uses, fixing the CI failure of f4aacb5.

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

web/checkout.js

@@ -112,12 +112,21 @@
         return window.dtfUpload(file,{api,scope:session.cart_scope,progress:()=>{},
           onBytes:n=>{e.sent=n;pintaEnvio();}});
       })();
-      e.promise.then(()=>{e.done=true;pintaEnvio();},
-        error=>{e.failed=error;envios.delete(k);pintaEnvio();});
+      e.promise.then(()=>{e.done=true;falhas.delete(k);pintaEnvio();},
+        error=>{envios.delete(k);falhas.set(k,{file,error});pintaEnvio();});
       envios.set(k,e);
     }
     return e.promise;
   }
+  // A failed upload is shown and waits for the customer, never retried behind
+  // their back: a file the check refused would fail again on every change.
+  const falhas=new Map();
+  function tentaDeNovo() {
+    const arquivos=[...falhas.values()].map(f=>f.file);
+    falhas.clear();
+    arquivos.forEach(f=>enviar(f).catch(()=>{}));
+    pintaEnvio();
+  }
   const arquivosDoCarrinho=()=>[...pedido,...(busy&&itemAtual?[itemAtual]:[])].flatMap(it=>it.localFiles||[]);
   const gb=n=>(n/1073741824).toLocaleString('pt-BR',{maximumFractionDigits:1})+' GB';
   const mb=n=>n>=1073741824 ? gb(n) : Math.round(n/1048576)+' MB';
@@ -149,6 +158,20 @@
   function pintaEnvio() {
     const e=estadoEnvio(), texto=textoEnvio(e);
     const el=document.getElementById('envioArq');
+    const falhou=arquivosDoCarrinho().map(f=>falhas.get(chaveArquivo(f))).filter(Boolean);
+    const topo=document.getElementById('cartEnvio');
+    if (falhou.length) {
+      if (el) {
+        el.replaceChildren(document.createTextNode('Não foi possível enviar '+falhou[0].file.name+': '+
+          (falhou[0].error?.message||'erro no envio').replace(/\.$/,'')+(falhou.length>1?' (e mais '+(falhou.length-1)+')':'')+'. '));
+        const b=document.createElement('button'); b.type='button'; b.className='envioDeNovo'; b.textContent='Tentar de novo';
+        b.onclick=tentaDeNovo; el.append(b);
+        el.classList.remove('ok'); el.classList.add('erro');
+      }
+      if (topo) topo.textContent='· falha no envio';
+      return;
+    }
+    if (el) el.classList.remove('erro');
     if (el) {
       el.replaceChildren();
       if (e && e.fase!=='pronto') {
@@ -160,7 +183,6 @@
       el.classList.toggle('ok', e?.fase==='pronto');
     }
     // Visible on every page while it runs.
-    const topo=document.getElementById('cartEnvio');
     if (topo) topo.textContent = !e || e.fase==='pronto' ? '' : e.fase==='verificando' ? '· verificando' : '· enviando '+e.pct+'%';
     if (busy && texto) {
       message(texto);
@@ -173,7 +195,10 @@
     if (e && e.fase==='enviando') { event.preventDefault(); event.returnValue=''; }
   });
   // Only what is in the cart: the item on the product page may still change.
-  window.addEventListener('dtf-cart-changed',()=>{ arquivosDoCarrinho().forEach(f=>{ enviar(f).catch(()=>{}); }); pintaEnvio(); });
+  window.addEventListener('dtf-cart-changed',()=>{
+    arquivosDoCarrinho().forEach(f=>{ if(!falhas.has(chaveArquivo(f))) enviar(f).catch(()=>{}); });
+    pintaEnvio();
+  });
   async function upload(file) {
     return enviar(file);
   }