Replace the video-pic/video/clear button labels with icon-only buttons
carrying title + aria-label (设置视频封面/视频/清除), center the glyphs
via flex to fix the upward icon shift, rename the media picker title to
为视频设置封面, and drop the unused setVideoCover i18n key. Contract
test updated to pin the new markup and string set.
Featured video preview now renders a real DPlayer (video-url/video-pic)
like the reference theme instead of a static image, so the cover video
plays on the submit page. Top-right bar gains the video-cover button
shown only when a video cover is loaded; backend honors pic_id and no
longer deletes the thumbnail before cover validation. Drop dead i18n
keys and data-can-* attributes.
kate522