[Feature]: embed local images as base64/data URLs in exported HTML

When an exported HTML deck references local image paths, the images only work on the original machine. After sharing the HTML file with others, those images are broken because the local files are not available.

It would be useful to support converting local images into base64/data URLs during HTML export, so the output becomes a fully self-contained file that can be shared directly.

Remote http(s) image URLs can remain unchanged.

If you think this makes sense, I’d also be happy to contribute code to help implement it.