fix: make mobile ToC sticky so it stays visible while scrolling

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: braboj

astro-site/src/styles/global.css

@@ -595,6 +595,10 @@ a:hover {
     border: 1px solid var(--color-border);
     border-radius: 4px;
     padding: 0;
+    position: sticky;
+    top: 0;
+    z-index: 10;
+    background: var(--color-bg);
   }
 
   .mobile-toc summary {