feat(submit): fold slide cover into multi-select image picker

Match the featured cover buttons row to the reference theme: drop the
dedicated slide button, let the icon-only image picker select multiple
images into a slide cover (single pick stays an image cover), and align
the loaded-state visibility matrix.

assets/css/frontend/pages/submit.css

@@ -546,9 +546,10 @@ body.aksrt-editor-media-open {
   font-size: 1em;
 }
 
-/* 纯图标按钮（设置视频封面/视频/清除）：fa 字形行高与文字不同导致图标上偏，
+/* 纯图标按钮（图片/设置视频封面/视频/清除）：fa 字形行高与文字不同导致图标上偏，
    用 flex 居中修正；并去掉 legacy .but 给图标加的文字间距。btn-delete 的
    hidden 属性切换依赖默认 display，故用 :not([hidden]) 守卫。 */
+.featured-edit .btns-full .btn-image,
 .featured-edit .btns-full .btn-video,
 .featured-edit .btns-full .btn-delete:not([hidden]) {
   display: inline-flex;
@@ -566,6 +567,7 @@ body.aksrt-editor-media-open {
   justify-content: center;
 }
 
+.featured-edit .btns-full .btn-image > .fa,
 .featured-edit .btns-full .btn-video-pic > .fa,
 .featured-edit .btns-full .btn-video > .fa,
 .featured-edit .btns-full .btn-delete > .fa {
@@ -573,11 +575,8 @@ body.aksrt-editor-media-open {
 }
 
 .featured-loaded.image .btn-video,
-.featured-loaded.image .btn-slide,
 .featured-loaded.slide .btn-video,
-.featured-loaded.slide .btn-image,
-.featured-loaded.video .btn-image,
-.featured-loaded.video .btn-slide {
+.featured-loaded.video .btn-image {
   display: none;
 }