fix(video-cover): size the swapped <video> like the cover <img> (post + list)
The iOS swap replaces the cover <img> with a <video>, but the post-page and list-cover CSS target
.post-cover img / .post-list-cover img (the element), so the <video> inherited no sizing and
rendered at its intrinsic height — too tall for the width. Add video to those selectors. Grid +
related covers already work (the img's class travels to the video, and those rules are class-based).
Co-Authored-By: Claude <noreply@…>