Release 1.1.18: fix bootstrap admin lockout, refresh screenshots

models/user_redis.js hardcoded the bootstrap anti-lockout admin
username to 'proxyadmin2', while migrations/permission_bootstrap.js
grants the global-admin permission to conf.auth.adminUsers[0]. An
operator who customized adminUsers away from the default ended up
with a bootstrapped account that had no admin permissions -- a
silent lockout. user_redis.js now derives the bootstrap username the
same way permission_bootstrap.js does.

Also corrected a secrets.js.example comment that claimed the
bootstrap password defaults to the username itself (it actually
generates and logs a random password), and refreshed all README
screenshots against the current UI, including a new load-balancing
screenshot.

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

README.md

@@ -31,6 +31,10 @@ management once basic auth is enabled:
 
 [![Per-host basic auth](docs/images/host-auth-basic.png)](docs/images/host-auth-basic.png)
 
+Multiple backend targets per host, load balanced round-robin:
+
+[![Load balancing](docs/images/load-balancing.png)](docs/images/load-balancing.png)
+
 ## Why this over the alternatives
 
 Nginx Proxy Manager, Traefik, and Caddy are all good reverse proxies with