Keep WebP animating on iOS by attaching the source img to the DOM

iOS pauses animation on a detached <img>, so the CanvasTexture sampled a
frozen frame. Attach the img hidden off-screen (opacity 0.01) so WebKit
keeps advancing the WebP; remove it on clear.

public/preview.html

@@ -253,6 +253,10 @@
         // the texture animates and transparency is preserved.
         const img = document.createElement('img');
         img.crossOrigin = 'anonymous'; img.src = image;
+        // iOS pauses animation on a detached <img>, so attach it hidden off-screen
+        // to keep the WebP advancing. We sample its current frame into the canvas.
+        img.style.cssText = 'position:fixed;left:-99999px;top:0;width:64px;height:64px;pointer-events:none;opacity:0.01;';
+        document.body.appendChild(img);
         const cnv = document.createElement('canvas');
         cnv.width = 256; cnv.height = 256;
         const ctx = cnv.getContext('2d');
@@ -301,6 +305,7 @@
     function clearGhost() {
       if (!current) return;
       if (current.userData.vidEl) { try { current.userData.vidEl.pause(); current.userData.vidEl.src = ''; } catch {} }
+      if (current.userData.animImg) { try { current.userData.animImg.remove(); } catch {} }
       scene.remove(current);
       current = null;
     }