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>

src/App.tsx

@@ -1364,7 +1364,7 @@ function LandingPage({ content }: { content: SiteContent }) {
       <section className="section-home-jump" aria-label="Homepage navigation">
         <div className={`section-inner home-ways${featuredStudy ? '' : ' home-ways--list-only'}`}>
           {featuredStudy && (
-            <Link to={`/study/${featuredStudy.slug || ''}`} className="home-feature">
+            <Link to={`/study/${featuredStudy.slug || ''}`} className={`home-feature${featuredStudy.imageUrl ? ' home-feature--image' : ''}`} style={featuredStudy.imageUrl ? { backgroundImage: `linear-gradient(180deg, rgba(10, 10, 8, 0.15) 0%, rgba(10, 10, 8, 0.55) 45%, rgba(10, 10, 8, 0.94) 100%), url("${featuredStudy.imageUrl}")` } : undefined}>
               <div className="home-feature-head">
                 <span className="home-feature-eyebrow">{featuredStudy.homepageEyebrow || 'Study'}</span>
                 {featuredStudy.showNewTag && <span className="home-jump-new-tag">{featuredStudy.newTagLabel || 'NEW'}</span>}