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/sw.js

@@ -1,5 +1,5 @@
 // App-shell cache only; Seafile/API traffic always goes to the network.
-const CACHE = 'hub-v51';
+const CACHE = 'hub-v52';
 const SHARE = 'hub-share'; // files shared into the app from other apps, held until the person picks a folder
 const SHELL = ['/icon.svg', '/manifest.webmanifest', '/icons/icon-192.png', '/icons/apple-touch-icon.png'];