fix(header): use explicit popover="auto" instead of boolean attribute

Hono JSX renders bare `popover` as `popover="true"` which relies on
the invalid-value-default behavior. Explicit "auto" is clearer.

Author: danielnaab

src/design/components/header/index.tsx

@@ -27,7 +27,7 @@ export function Header({ config }: { config: SiteConfig }) {
         </svg>
       </button>
       <nav aria-label="Primary">
-        <div id="mobile-nav" popover>
+        <div id="mobile-nav" popover="auto">
           <ul>
             <li>
               <a href={url('/work/', config.basePath)}>Work</a>