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/docker/nginx-demo.conf

@@ -62,6 +62,14 @@ http {
             proxy_set_header X-Real-IP $remote_addr;
         }
 
+        # ElectrumX UI status (polled by the electrs-ui shell)
+        location /electrs-status {
+            proxy_pass http://neode-backend:5959;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+        }
+
         # Proxy FileBrowser API to mock backend (demo mode)
         location /app/filebrowser/ {
             client_max_body_size 10G;