use serial, not date, as sort key

lib/generate-feed.php

@@ -17,7 +17,7 @@ function generateFeed($unsortedContent) {
 
     $content = $unsortedContent;
     // Sort by date descending (newest first)
-    usort($content, fn($a, $b) => $a->pubDate < $b->pubDate ? 1 : -1);
+    usort($content, fn($a, $b) => $a->serial < $b->serial ? 1 : -1);
 
     foreach ($content as $page) {
         $content = $twig->render('item-content.html', [