fix(demo): mock backend overhaul — real media, correct shapes, mempool.guide

- Serve committed demo/content (music/photos/documents) alongside demo/files
  drop-ins, normalizing top-level folder names; Dockerfile now COPYs it. This
  is why Cloud music failed: the real library sat in demo/content while the
  loader only read the empty demo/files.
- Drop unbacked seeded Videos; runtime WAV/SVG fallback for any seeded media
  without a real file so playback never hits 'no supported source'.
- monitoring.current/history/alerts/alert-rules/export rewritten to the exact
  MetricSnapshot shapes Monitoring.vue expects (epoch-second timestamps,
  containers with limits/block IO, rpc_latency_ms, ws_connections).
- streaming.list-services + configure-service (Networking Profits page was
  erroring with Method not found).
- server.get-state snapshot so the 30s resync / reconnect path stops erroring.
- lnd-connect-info now returns cert/macaroon base64url + grpc/rest ports (the
  lnd-ui shell only renders the wallet QR + details when they're present);
  LND REST balance endpoints for the shell's new balance cards.
- Fix broken icon paths (lnd.svg → lnd.png and 15 others) against real assets.
- containers-scanned: true so app cards never sit on 'Checking…'.
- 8 new installed demo apps with placeholder dashboard UIs (btcpay-server,
  grafana, nextcloud, jellyfin, vaultwarden, nostr-rs-relay, searxng,
  uptime-kuma) via the previously unused demoAppShell.
- WS heartbeat 45s → 20s (+ping 25s) so idle-timeout proxies in front of the
  public demo stop killing the socket (the 'always reconnecting' report).
- nginx demo proxy: mempool.space → mempool.guide (mempool.space blocks the
  proxied embed) + WebSocket upgrade passthrough; txid hydration follows.

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

neode-ui/Dockerfile.backend

@@ -15,12 +15,14 @@ RUN npm install
 COPY neode-ui/ ./
 
 # Sibling assets the mock backend reads relative to /app (../docker, ../demo):
-# the Bitcoin UI mock shell and any curated cloud files dropped into demo/files.
+# the Bitcoin UI mock shell and the curated cloud files (demo/files drop-ins +
+# the committed demo/content library — both are scanned by loadDemoDiskFiles).
 COPY docker/bitcoin-ui /docker/bitcoin-ui
 COPY docker/electrs-ui /docker/electrs-ui
 COPY docker/lnd-ui /docker/lnd-ui
 COPY docker/fedimint-ui /docker/fedimint-ui
 COPY demo/files /demo/files
+COPY demo/content /demo/content
 
 # Expose port
 EXPOSE 5959