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.

.gitignore

@@ -2,6 +2,7 @@
 /.git/
 /.claude/
 /.compare/
+/.zcode/
 
 # Project planning, internal docs, and local-only development files
 /.specs/
@@ -52,6 +53,9 @@
 !/tests/widget-system/circular-button-icon-margin-contract.test.cjs
 !/tests/widget-system/single-cover-height-contract.test.cjs
 !/tests/widget-system/frontend-lifecycle-contract.test.cjs
+!/tests/front-submit/
+/tests/front-submit/*
+!/tests/front-submit/video-cover-contract.test.cjs
 !/tests/asset-ledger/
 !/tests/asset-ledger/*.php
 !/tests/visual/docker-compose.zibll.yml