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.css

@@ -3405,18 +3405,18 @@
     flex-shrink: 0;
   }
 
-  .section-resources .resource-link {
+  .section-external-sites .resource-link {
     display: flex;
     flex-direction: column;
     align-items: flex-start;
     min-height: unset;
   }
 
-  .section-resources .resource-download-meta {
+  .section-external-sites .resource-download-meta {
     width: 100%;
   }
 
-  .section-resources .resource-link-image {
+  .section-external-sites .resource-link-image {
     width: 100%;
     height: 160px;
     border-radius: 10px;