refactor: move header defaults into ds utilities

docs/kitchen-sink.html

@@ -8,7 +8,7 @@
   </head>
   <body class="bg-page font-sans text-text-strong">
     <div class="min-h-screen bg-page-alt">
-      <header class="bg-surface text-text shadow-modal">
+      <header class="ds-header bg-surface text-text shadow-modal">
         <div
           class="mx-auto flex max-w-6xl items-center justify-between gap-sm px-md py-sm"
         >