Add build-time cache-busting query string to main CSS (#193)

Appends the Jekyll build timestamp to the CSS URL so browsers
always fetch fresh styles after a deployment.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: mmcky

_layouts/default.html

@@ -47,7 +47,7 @@
   <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/swiper@8/swiper-bundle.min.css" />
 
   <!-- Template Main CSS File -->
-  <link href="/assets/main.css" rel="stylesheet">
+  <link href="/assets/main.css?v={{ site.time | date: '%Y%m%d%H%M%S' }}" rel="stylesheet">
   <link rel="icon" href="{{ '/assets/img/favicon.ico' | relative_url }}" type="image/x-icon" />
 </head>