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>

.dockerignore

@@ -7,10 +7,11 @@
 # Allow demo assets (AIUI pre-built dist)
 !demo/
 
-# Allow the Bitcoin UI mock shell (backend serves it from /docker/bitcoin-ui)
+# Allow the Bitcoin UI + ElectrumX UI mock shells (served from /docker/*)
 !docker/
 docker/*
 !docker/bitcoin-ui/
+!docker/electrs-ui/
 
 # Allow backend source for ISO source builds
 !core/