Keep the audiobooks frame alive when switching tabs

The frame was rebuilt with the page content on every route change, which
stopped playback. It now lives outside the view and is only shown or hidden.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

public/index.html

@@ -17,7 +17,7 @@
 <body>
 <div id="shell">
   <nav id="nav" aria-label="Main"></nav>
-  <div id="main"><main id="view"></main><div id="player" hidden></div></div>
+  <div id="main"><main id="view"></main><div id="audiowrap" class="off"></div><div id="player" hidden></div></div>
 </div>
 <div id="toast" role="status"></div>
 <script src="/webauthn.js"></script>