refactor(creator-studio): pick the cover from the toolbar image dialog

The cover row carried its own hidden file input and upload call, so authors
had two different image flows in one page.

- 添加封面 now opens window.JioSubmitMedia with type=image, the same dialog
  the toolbar's 图片 button uses (mine / upload / external link).
- The picked image is stored as the full-size URL plus its attachment id,
  matching FrontSubmitFeatured's image branch, which re-checks ownership
  and sets the post thumbnail.
- Removes the now-unused uploadCover path, apiUpload/nonce imports, and the
  .jio-creator__cover-input rule.

Contract added; 887 widget-system tests pass.

assets/creator-studio/build/index.asset.php

@@ -1 +1 @@
-<?php return array('dependencies' => array('react', 'wp-element', 'wp-i18n'), 'version' => '16333cda79ebcc6962cd');
+<?php return array('dependencies' => array('react', 'wp-element', 'wp-i18n'), 'version' => '0b6cb59c50798e13e6f5');