MediaStylePanel's Slides array editor guarded on item.image !== undefined
and wrote `image` via AssetPicker's onChange, but ContentSlider (render
+ toHtml) reads slide.imageSrc. Default slides (imageSrc:'', no `image`
key) never showed an image picker at all, and any `image` value written
was a silent no-op on render/export.
Editor now guards/reads/writes `imageSrc` throughout, and the
"add slide" emptyItem matches defaultSlides' exact shape
(type/imageSrc/heading/text/bgColor).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>