Scope external site link styles to homepage section only

The .section-resources CSS overrides were breaking the Downloads page
resource link layout. Added .section-external-sites class to the
ExternalSitesSection and scoped all image/layout overrides to it.

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

src/App.tsx

@@ -1106,7 +1106,7 @@ function ExternalSitesSection({ content }: { content: SiteContent }) {
   if (links.length === 0) return null
 
   return (
-    <section className="section-resources" aria-label="External sites">
+    <section className="section-resources section-external-sites" aria-label="External sites">
       <div className="section-inner">
         <div className="download-library-head">
           <p className="eyebrow">Homepage Links</p>