perf: streamline Google Fonts loading

- Removed redundant Inter font request (used as a fallback)
- Simplified font weights for Shantell Sans and Plus Jakarta Sans to only request used weights
- Reduced overall CSS payload and external requests for better First Contentful Paint (FCP)

Author: iasonliu

_layouts/default.html

@@ -9,7 +9,7 @@
     <!-- Google Fonts -->
     <link rel="preconnect" href="https://fonts.googleapis.com">
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
-    <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=JetBrains+Mono:wght@400;500;600&family=Noto+Serif+SC:wght@400;500;600;700&family=Plus+Jakarta+Sans:wght@500;600;700;800&family=Shantell+Sans:ital,wght@0,400;0,500;0,600;1,400&family=Source+Serif+4:ital,opsz,wght@0,8..60,400;0,8..60,500;0,8..60,600;1,8..60,400&display=swap" rel="stylesheet">
+    <link href="https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@400;500;600&family=Noto+Serif+SC:wght@400;500;600;700&family=Plus+Jakarta+Sans:wght@400;500;600;700&family=Shantell+Sans:wght@400;500;600;700&family=Source+Serif+4:ital,opsz,wght@0,8..60,400;0,8..60,500;0,8..60,600;1,8..60,400&display=swap" rel="stylesheet">
 
     <!-- Stylesheets (with cache-busting timestamp) -->
     <link rel="stylesheet" href="/css/syntax.css?v={{ site.time | date: '%s' }}" type="text/css" />