feat: error page, admin-only DNS page, navbar active styling (v1.33.0)

- Add SSO-style error page (views/error.ejs) and render it for browser
  navigation in the error handler (API still returns JSON).
- DNS page admin-only: forceLogin(['admin']) + nav groups ['admin'].
- Navbar: username not underlined; only the active nav link is bold+underlined.

Co-Authored-By: Claude <noreply@anthropic.com>

nodejs/views/dns.ejs

@@ -1,7 +1,7 @@
 <%- include('top') %>
 <script type="text/javascript">
-	// Require login to see this page.
-	app.auth.forceLogin();
+	// Require an admin to see this page.
+	app.auth.forceLogin(['admin']);
 </script>
 
 <style type="text/css">