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.css

@@ -13904,3 +13904,7 @@ a.study-track-card:hover {
   .home-ways { grid-template-columns: 1fr; }
   .home-feature { min-height: 260px; padding: 1.6rem; }
 }
+
+.home-feature--image { background-size: cover; background-position: center; }
+.home-feature--image h3 { text-shadow: 0 2px 12px rgba(0, 0, 0, 0.6); }
+.home-feature--image p { color: #d9ceb0; }