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.

assets/css/frontend/pages/submit.css

@@ -498,23 +498,13 @@ body.aksrt-editor-media-open {
   display: none;
 }
 
-.featured-edit .featured-preview > img,
-.featured-edit .featured-video-placeholder {
+.featured-edit .featured-preview > img {
   display: block;
   width: 100%;
   aspect-ratio: 2 / 1;
   object-fit: cover;
 }
 
-.featured-edit .featured-video-placeholder {
-  display: flex;
-  align-items: center;
-  justify-content: center;
-  flex-direction: column;
-  gap: 8px;
-  color: var(--muted-2-color, #999);
-}
-
 .featured-edit .featured-type-badge {
   position: absolute;
   right: 12px;
@@ -557,6 +547,14 @@ body.aksrt-editor-media-open {
   font-size: 1em;
 }
 
+.featured-edit .btns-full .btn-video-pic {
+  display: none;
+}
+
+.featured-loaded.video .btns-full .btn-video-pic {
+  display: inline-flex;
+}
+
 .featured-loaded.image .btn-video,
 .featured-loaded.image .btn-slide,
 .featured-loaded.slide .btn-video,