Add design system feature flag and runtime integration

components/iframe_forms/iframe-application-form.html

@@ -11,7 +11,7 @@
     <!-- Load nostr‑tools v2.10.4 -->
     <script src="https://cdn.jsdelivr.net/npm/[email protected]/lib/nostr.bundle.min.js"></script>
   </head>
-  <body>
+  <body data-ds="legacy">
     <div class="container">
       <div class="form-container">
         <p>