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'),

templates/parts/submit/featured.php

@@ -22,10 +22,7 @@ $can_slide = !empty($ctx['can_cover_slide']);
 $can_video = !empty($ctx['can_cover_video']);
 ?>
 <div class="featured-edit<?php echo $type !== '' ? ' featured-loaded ' . esc_attr($type) : ''; ?>"
-	data-featured-args="<?php echo esc_attr(wp_json_encode($args)); ?>"
-	data-can-image="<?php echo $can_image ? '1' : '0'; ?>"
-	data-can-slide="<?php echo $can_slide ? '1' : '0'; ?>"
-	data-can-video="<?php echo $can_video ? '1' : '0'; ?>">
+	data-featured-args="<?php echo esc_attr(wp_json_encode($args)); ?>">
 	<div class="featured-preview" data-featured-preview>
 		<?php if ($type === 'image' && !empty($data['url'])) : ?>
 			<img src="<?php echo esc_url((string) $data['url']); ?>" alt="">
@@ -33,11 +30,13 @@ $can_video = !empty($ctx['can_cover_video']);
 			<img src="<?php echo esc_url((string) $data[0]['url']); ?>" alt="">
 			<span class="featured-type-badge"><i class="fa fas fa-image"></i></span>
 		<?php elseif ($type === 'video') : ?>
-			<?php if (!empty($data['pic'])) : ?>
-				<img src="<?php echo esc_url((string) $data['pic']); ?>" alt="">
-			<?php else : ?>
-				<div class="featured-video-placeholder"><i class="fa fas fa-play"></i></div>
-			<?php endif; ?>
+			<div class="new-dplayer dplayer-scale-height" style="--scale-height:50%;"
+				video-url="<?php echo esc_url((string) ($data['url'] ?? '')); ?>"
+				video-pic="<?php echo esc_url((string) ($data['pic'] ?? '')); ?>">
+				<div class="graphic" style="padding-bottom:50%;">
+					<div class="abs-center text-center"><i class="fa fas fa-play-circle fa-4x muted-3-color opacity5" aria-hidden="true"></i></div>
+				</div>
+			</div>
 			<span class="featured-type-badge"><i class="fa fas fa-video"></i></span>
 		<?php endif; ?>
 	</div>
@@ -47,6 +46,11 @@ $can_video = !empty($ctx['can_cover_video']);
 				<i class="fa fas fa-image"></i> <?php esc_html_e('图片', 'aksrt'); ?>
 			</button>
 		<?php endif; ?>
+		<?php if ($can_video) : ?>
+			<button type="button" class="but btn-video-pic" data-featured-action="video_pic">
+				<i class="fa fas fa-image"></i> <?php esc_html_e('设置视频首图封面', 'aksrt'); ?>
+			</button>
+		<?php endif; ?>
 		<?php if ($can_slide) : ?>
 			<button type="button" class="but btn-slide" data-featured-action="slide">
 				<i class="fa fas fa-image"></i> <?php esc_html_e('幻灯片', 'aksrt'); ?>