Offline shell (precache, offline notice) and accessibility layer (skip link, button names, sheet focus and Escape)

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

public/index.html

@@ -56,5 +56,6 @@
 <script src="/links.js"></script>
 <script src="/palette.js"></script>
 <script src="/app.js"></script>
+<script src="/shell.js"></script>
 </body>
 </html>