feat(submit): add video cover picker with playable video preview

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.

templates/pages/page-submit.php

@@ -147,9 +147,8 @@ if (file_exists($js)) {
 			'fileTooLarge'  => __('文件大小超过限制', 'aksrt'),
 			'lastSaved'     => __('最后保存：', 'aksrt'),
 			'preview'       => __('预览文章', 'aksrt'),
-			'pickImage'     => __('选择图片', 'aksrt'),
-			'pickVideo'     => __('选择视频封面图', 'aksrt'),
-			'videoUrl'      => __('请输入视频地址', 'aksrt'),
+			'setVideoCover' => __('设置视频首图封面', 'aksrt'),
+			'setVideoCoverTitle' => __('为视频设置首图封面', 'aksrt'),
 			'close'         => __('关闭', 'aksrt'),
 			'insertImage'   => __('插入图片', 'aksrt'),
 			'uploadImage'   => __('上传图片', 'aksrt'),
@@ -198,8 +197,6 @@ if (file_exists($js)) {
 			'maxMediaItems'  => __('最多选择 %d 个文件', 'aksrt'),
 			'uploadingClose' => __('上传中，请稍候再关闭', 'aksrt'),
 			'mediaPickerUnavailable' => __('媒体选择器加载失败，请刷新页面后重试', 'aksrt'),
-			'pickVideoResource' => __('选择视频资源', 'aksrt'),
-			'pickVideoCover' => __('选择视频封面（可选）', 'aksrt'),
 			'download'       => __('下载', 'aksrt'),
 
 			'quote'          => __('引言', 'aksrt'),