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.

.gitignore

@@ -56,6 +56,7 @@
 !/tests/front-submit/
 /tests/front-submit/*
 !/tests/front-submit/video-cover-contract.test.cjs
+!/tests/front-submit/featured-buttons-contract.test.cjs
 !/tests/front-submit/editor-fullscreen-contract.test.cjs
 !/tests/asset-ledger/
 !/tests/asset-ledger/*.php