Show image on external site link cards on homepage

Adds the configured imageUrl as a full-width banner at the top of each
External Sites card, above the link name and description.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.tsx

@@ -1117,6 +1117,7 @@ function ExternalSitesSection({ content }: { content: SiteContent }) {
           {links.map(link => (
             <a key={link.id} href={link.url} target="_blank" rel="noreferrer" className="resource-link">
               <div className="resource-download-meta">
+                {link.imageUrl && <img src={link.imageUrl} alt={link.label} className="resource-link-image" />}
                 <span className="resource-link-label">{link.label}</span>
                 {link.description && <p className="resource-link-description">{link.description}</p>}
                 <span className="resource-link-action">Open external site →</span>