feat(demo): black-theme app UIs w/ icons, real ElectrumX UI, Core/Knots split

- Mock app UIs (ElectrumX, LND, Fedimint, Bitcoin Core) + the "Not available"
  notice now use the Archipelago black theme and show the app's My-Apps icon.
- Bitcoin Core gets its own UI (/app/bitcoin-core/) so it no longer shows Bitcoin
  Knots branding; the Knots-branded bitcoin-ui shell is reserved for Bitcoin Knots.
- ElectrumX now serves the real electrs-ui shell (+ qrcode.js + a dummy
  /electrs-status) with the correct ElectrumX icon; "Electrs" renamed to ElectrumX.
- My Apps: pre-install Bitcoin Knots again, drop ThunderHub, rename Electrs→ElectrumX.
- App store no longer shows "Checking…" forever in demo — non-demoable apps show
  "No demo" immediately (skip the container-scan state).
- Relay endpoint no longer reveals a real domain (randomised host).
- Dockerfile/.dockerignore copy docker/electrs-ui into the backend image.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

neode-ui/src/composables/useDemoIntro.ts

@@ -62,8 +62,8 @@ const DEMO_EXTERNAL_URLS: Record<string, string> = {
 // Apps with a same-origin mock UI served by the demo backend.
 const DEMO_MOCK_UI: Record<string, string> = {
   'bitcoin-knots': '/app/bitcoin-ui/',
-  'bitcoin-core': '/app/bitcoin-ui/',
-  bitcoin: '/app/bitcoin-ui/',
+  'bitcoin-core': '/app/bitcoin-core/',
+  bitcoin: '/app/bitcoin-core/',
   'bitcoin-ui': '/app/bitcoin-ui/',
   electrs: '/app/electrumx/',
   electrumx: '/app/electrumx/',