Header: point Models link to Carbon collection

Re-applies the change from ea40f6c, which was silently dropped during
the rebase onto d001974 because the surrounding banner-links structure
had shifted.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

demo.html

@@ -195,7 +195,7 @@
              the report is published. -->
         <ul class="banner-links" aria-label="Resources">
           <li>
-            <a href="https://huggingface.co/HuggingFaceBio/Carbon-3B" target="_blank" rel="noopener">
+            <a href="https://huggingface.co/collections/HuggingFaceBio/carbon" target="_blank" rel="noopener">
               Models<span class="arrow" aria-hidden="true">↗</span>
             </a>
           </li>