fix(demo): /app proxy (fixes 404s), mempool iframe, LND UI, icons

- nginx-demo.conf + vite proxy now route every /app/<id>/ to the mock backend, so
  the per-app mock UIs and the generic "Not available in the demo" notice render
  (previously only /app/filebrowser was proxied → most apps 404'd).
- Mempool and IndeeHub now load in the in-app iframe (not a new tab).
- Add an LND Lightning mock UI (channels, balances, routing) with dummy data;
  lnd/thunderhub are demoable. Notice page reworded to "Not available in the demo".
- Fix missing icons: Bitcoin Core → bitcoin-core.png, Mempool → mempool.webp.
- Pre-install only Bitcoin Core (drop duplicate Bitcoin Knots; still installable).

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

neode-ui/docker/nginx-demo.conf

@@ -72,6 +72,16 @@ http {
             proxy_request_buffering off;
         }
 
+        # Proxy every other app UI (/app/<id>/) to the mock backend, which serves
+        # the per-app mock UIs (bitcoin-ui, electrumx, lnd, fedimint) and the
+        # generic "Not available in the demo" notice for the rest.
+        location /app/ {
+            proxy_pass http://neode-backend:5959;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+        }
+
         # Serve AIUI SPA
         location /aiui/ {
             alias /usr/share/nginx/html/aiui/;