Studies: add a cover image setting, used as the backdrop of the featured study card on the homepage; v2.36.0

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server/helpers.js

@@ -341,6 +341,7 @@ function sanitizeStudies(value) {
       title: typeof item.title === 'string' ? item.title.trim().slice(0, 160) : '',
       description: typeof item.description === 'string' ? item.description.trim().slice(0, 800) : '',
       homepageEyebrow: typeof item.homepageEyebrow === 'string' ? item.homepageEyebrow.trim().slice(0, 80) : '',
+      imageUrl: sanitizeUrl(item.imageUrl) || '',
       showOnHomepage: item.showOnHomepage === true,
       showNewTag: item.showNewTag === true,
       newTagLabel: typeof item.newTagLabel === 'string' ? item.newTagLabel.trim().slice(0, 24) : '',