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>

docs/extensions/notepad.html

@@ -0,0 +1,5 @@
+<!doctype html><html><head><meta charset="utf-8"><title>Notepad</title><style>body{font-family:sans-serif;margin:12px}textarea{width:100%;height:200px}</style></head><body><h3>Notepad</h3><textarea id="t"></textarea><p id="who"></p><script>
+hub.user().then(function(u){document.getElementById('who').textContent='Hello '+u.name});
+hub.storage.get('note').then(function(v){document.getElementById('t').value=v||''});
+document.getElementById('t').oninput=function(e){hub.storage.set('note',e.target.value)};
+</script></body></html>