feat(submit): make video cover row icon-only with hover titles

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.

assets/js/front-submit.js

@@ -522,7 +522,7 @@
             type: 'image',
             multiple: false,
             allowExternal: false,
-            title: i18n.setVideoCoverTitle || '为视频设置首图封面',
+            title: i18n.setVideoCoverTitle || '为视频设置封面',
             onSelect: function (cover) {
               self.setFeatured('video', {
                 url: current.data.url,
@@ -554,7 +554,7 @@
                   type: 'image',
                   multiple: false,
                   allowExternal: false,
-                  title: i18n.setVideoCoverTitle || '为视频设置首图封面',
+                  title: i18n.setVideoCoverTitle || '为视频设置封面',
                   onSelect: function (cover) {
                     self.setFeatured('video', {
                       url: video.url,