Preview pane: hide Print / Save PDF on pixel-art designs

Pixel-art designs already expose their own one-click Download
PNG / GIF button (commit 95a39cc), which produces the right asset
for raster sprites at the right native scale. Print / Save PDF
on top of that is redundant + confusing — a printed PDF of a
32x32 pixel sprite isn't a useful artefact.

Gate the printBtn injection on mode !== 'pixelart'. All other
modes (cover, slide, deck, customart, jewel, disc-design, disc)
keep the button as before.

src/index.html

@@ -22387,7 +22387,10 @@
 
         actionsDiv.insertBefore(import1Btn, saveJsonBtn);
         actionsDiv.insertBefore(import2Btn, saveJsonBtn);
-      } else {
+      } else if (mode !== 'pixelart') {
+        // Pixel-art designs have their own PNG / GIF download button
+        // a few lines up — Print / Save PDF doesn't make sense for
+        // raster sprites at small native resolutions.
         const printBtn = document.createElement('button');
         printBtn.type = 'button';
         printBtn.textContent = 'Print / Save PDF';