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/music.js

@@ -13,7 +13,7 @@ const px = {
     if (!r.ok) throw new Error('Plex error ' + r.status);
     return (await r.json()).MediaContainer || {};
   },
-  art: (thumb, w = 300) => thumb && /^\/api\/(podcast|ab|radio|vbv)\//.test(thumb) ? thumb : thumb ? `/plex/photo/:/transcode?width=${w}&height=${w}&minSize=1&upscale=1&url=${encodeURIComponent(thumb)}` : 'data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==',
+  art: (thumb, w = 300) => thumb && /^\/api\/(podcast|ab|radio|vbv|store)\//.test(thumb) ? thumb : thumb ? `/plex/photo/:/transcode?width=${w}&height=${w}&minSize=1&upscale=1&url=${encodeURIComponent(thumb)}` : 'data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==',
   // All music libraries on the server; the chosen one is remembered per server.
   async sections() {
     if (px.secs) return px.secs;