Add plan page to admin nav

public/admin/admin.js

@@ -39,7 +39,7 @@ export const styles = `
 export function nav(active) {
   return `<header>
     <h1>Newbury Exhibit — Admin</h1>
-    ${['layout', 'playlist', 'print', 'calibrate', 'errors'].map(p =>
+    ${['layout', 'plan', 'playlist', 'print', 'calibrate', 'errors'].map(p =>
       `<a href="/admin/${p}.html" class="${p === active ? 'active' : ''}">${p}</a>`).join('')}
     <a href="/">viewer ↗</a>
   </header>`;