Store: one tab combining books, free audiobooks and radio, plus My Store for your own books, music, apps and sandboxed extensions

- Store tab with sections; the book store, LibriVox and radio finder are shown inside it (old entry points redirect)
- My Store: admin uploads files or copies from Seafile; books go to a person's shelf, music plays in the player, apps download
- Extensions: one HTML file run in a sandboxed frame with a small storage/toast/user bridge; example in docs/extensions

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

public/index.html

@@ -76,6 +76,7 @@
 <script src="/audiobooks-view.js"></script>
 <script src="/librivox-view.js"></script>
 <script src="/radio-view.js"></script>
+<script src="/store-view.js"></script>
 <script src="/recipes-view.js"></script>
 <script src="/contacts-view.js"></script>
 <script src="/bible-view.js"></script>