fix: markdown thumbnail blank capture — replace visibility:hidden with off-screen positioning (#235)

html-to-image (toPng) clones the DOM node with computed styles intact,
so visibility:hidden produced a blank PNG. Use left:-9999 off-screen
positioning instead, matching the IframeCapture pattern.

Author: cjimti

ui/src/components/ThumbnailGenerator.tsx

@@ -215,14 +215,12 @@ function DomCapture({
       ref={containerRef}
       style={{
         position: "fixed",
-        left: 0,
-        top: 0,
+        left: -9999,
+        top: -9999,
         width: THUMB_WIDTH,
         height: THUMB_HEIGHT,
         overflow: "hidden",
         pointerEvents: "none",
-        visibility: "hidden",
-        zIndex: -1,
         background: "white",
         color: "black",
         fontSize: 12,