Fix Store tab stuck on Loading; adjustable idle time for family devices; videos count as in use

- A bare #/store now opens at #/store/books so the Book store page draws
- Admins choose how long a family device waits before returning to the board (1 to 30 minutes)
- Any playing video or audio keeps a family device signed in
- Test updated

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-v84';
+const CACHE = 'hub-v86';
 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'];