Share songs, albums, movies, shows, podcasts and audiobooks to chat as cards

A share message carries kind, title, subtitle, same-origin art and a hash-route
link, validated on the server. It counts as unread, reads as '🎵 Shared a song:
Title' in pushes and Notifications, and is found by search. In Chat a card opens
the item, or says it is not available when the recipient lacks that tab.

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-v90';
+const CACHE = 'hub-v91';
 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'];