fix: audit app icons — remove orphans, add missing nostrudel.svg

Removed orphaned icons: indeedhub.ico, community-store.png,
morphos-server.png, atob.png, k484.png. Created nostrudel.svg
placeholder. Cleaned mock-backend references.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

neode-ui/mock-backend.js

@@ -376,8 +376,6 @@ const marketplaceMetadata = {
   'nostr-rs-relay': { title: 'Nostr Relay', shortDesc: 'Run your own Nostr relay', icon: '/assets/img/app-icons/nostr-rs-relay.svg' },
   'syncthing': { title: 'Syncthing', shortDesc: 'Peer-to-peer file synchronization', icon: '/assets/img/app-icons/syncthing.png' },
   'tor': { title: 'Tor', shortDesc: 'Anonymous communication over the Tor network', icon: '/assets/img/app-icons/tor.png' },
-  'atob': { title: 'A to B Bitcoin', shortDesc: 'Bitcoin tools for seamless transactions', icon: '/assets/img/atob.png' },
-  'k484': { title: 'K484', shortDesc: 'Point of Sale and Admin system', icon: '/assets/img/k484.png' },
   'amin': { title: 'Amin', shortDesc: 'Administrative interface for Archipelago', icon: '/assets/icon/pwa-192x192-v2.png' },
 }