fix: handle null category in DocCard for versioned docs

Add optional chaining to prevent TypeError when category is null
in versioned docs context. The findPermalink function returns null
when the permalink isn't found in global metadata for versioned docs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: milldr

src/theme/DocCard/index.js

@@ -99,7 +99,7 @@ function CardCategory({item}) {
       href={href}
       icon="🗃️"
       title={`${item.label} (${item.items.length})`}
-      description={item.description ?? category.description ?? categoryItemsPlural(item.items.length)}
+      description={item.description ?? category?.description ?? categoryItemsPlural(item.items.length)}
     />
   );
 }