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.

tests/widget-system/creator-studio-contract.test.cjs

@@ -412,6 +412,20 @@ test('TinyMCE media buttons reuse Jio uploads and preserve bookmarks', () => {
 	assert.match(appSource, /<PublishSettings/);
 });
 
+// 封面复用工具栏「图片」按钮的媒体弹窗，不再自带一套上传 input。
+test('cover picker reuses the toolbar image media dialog', () => {
+	const settings = read('assets/creator-studio/src/components/publish-settings.tsx');
+	const styles = read('assets/creator-studio/src/styles/studio.scss');
+
+	assert.match(settings, /JioSubmitMedia/);
+	assert.match(settings, /type: 'image'/);
+	assert.match(settings, /onFeaturedChange\(\{ type: 'image'/);
+	// 旧的本地上传链路已经移除，不再有两套上传 UI。
+	assert.doesNotMatch(settings, /<input[^>]*type="file"/);
+	assert.doesNotMatch(settings, /apiUpload/);
+	assert.doesNotMatch(styles, /jio-creator__cover-input/);
+});
+
 test('writing surface is a centered paper card with a publish settings card below', () => {
 	const styles = read('assets/creator-studio/src/styles/studio.scss');
 	const settings = read('assets/creator-studio/src/components/publish-settings.tsx');