Fix: render error state in BufferRulesSection (fixes lint error)

The 'error' useState was declared but never read — only setError was called.
Now renders the error message as a red text node when the fetch fails.

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

src/components/BufferRules.tsx

@@ -144,6 +144,12 @@ export function BufferRulesSection() {
     );
   }
 
+  if (error) {
+    return (
+      <div className="py-4 text-sm text-red-500">{error}</div>
+    );
+  }
+
   return (
     <div>
       <div className="flex items-center justify-between mb-4">