Fix guide deep links, code block styling

- Move heading IDs from hidden permalink anchors to parent headings so deep links scroll correctly (admin.php)
- Change code block background from dark to light gray (guide.php)
- Add pre/code CSS to frontend layout.mustache for visible code blocks

public/admin.php

@@ -1417,6 +1417,14 @@ function handleGuide(AdminAuth $auth, array $config): void
         $environment->addExtension(new \League\CommonMark\Extension\HeadingPermalink\HeadingPermalinkExtension());
         $converter = new \League\CommonMark\MarkdownConverter($environment);
         $content = $converter->convert($rawContent)->getContent();
+
+        // Move IDs from heading-permalink anchors to their parent headings
+        // so deep links scroll to the heading element itself (not a hidden anchor)
+        $content = preg_replace(
+            '~<h(\d)>(.*?)<a id="([^"]+)"[^>]*></a></h\1>~s',
+            '<h\1 id="\3">\2</h\1>',
+            $content
+        );
     } else {
         $content = '<pre>' . htmlspecialchars($rawContent) . '</pre>';
     }