Docs: tested single-host install, NPM integration, installer --docker

- README: step-by-step installation of web UI and agent on one host,
  exactly as tested (agent after keygen, all "run" helpers before
  "up -d", http PUBLIC_URL and TRUSTED_PROXY_HOPS=0 without a proxy)
- README: attach to an existing Nginx Proxy Manager in Docker – shared
  network via docker-compose.override.yaml, no published port,
  gw_priority so SSH to the agent keeps the source address of the own
  network (otherwise from= rejects it), TLS settings, test
- install.sh --docker: the new manual jail bans in INPUT and DOCKER-USER
- Running container: helpers via "docker compose exec … python -m webui"
  (run fails with "Address already in use"); troubleshooting entries

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/installation.md

@@ -229,6 +229,13 @@ docker compose logs -f fail2ban-webui  # mit Strg+C verlassen
 Das Log zeigt eine Zusammenfassung wie
 `fail2ban-webui 1.0.0 – hosts: web01, lxc-proxy, dockerhost – auth: local – loki: off – geoip: on`.
 
+Ab jetzt läuft der Dienst. `docker compose run …` würde wegen der festen IP-Adresse mit
+`Address already in use` scheitern – Hilfsbefehle deshalb im laufenden Container aufrufen:
+
+```bash
+docker compose exec fail2ban-webui python -m webui check
+```
+
 Lokaler Test ohne Proxy:
 
 ```bash