fix(demo-ui): demoable apps launch in-frame; 'Not available in demo' label

- Register the 8 new placeholder apps in DEMO_MOCK_UI so they're launchable
  and installable in the demo.
- Demo bypasses the new-tab/tab-launch lists for demoable apps — they're
  served same-origin by the mock backend with no framing headers, so they
  render in the in-app session instead of opening broken localhost tabs.
- Non-demoable apps now say 'Not available in demo' instead of 'No demo'.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

neode-ui/src/composables/useDemoIntro.ts

@@ -75,6 +75,15 @@ const DEMO_MOCK_UI: Record<string, string> = {
   fedimint: '/app/fedimint/',
   fedimintd: '/app/fedimint/',
   filebrowser: '/app/filebrowser/',
+  // Static placeholder dashboards served by the mock backend (DEMO_APP_PAGES).
+  'btcpay-server': '/app/btcpay-server/',
+  grafana: '/app/grafana/',
+  nextcloud: '/app/nextcloud/',
+  jellyfin: '/app/jellyfin/',
+  vaultwarden: '/app/vaultwarden/',
+  'nostr-rs-relay': '/app/nostr-rs-relay/',
+  searxng: '/app/searxng/',
+  'uptime-kuma': '/app/uptime-kuma/',
 }
 
 /**