fix: use CAMELEER_API_URL env var for server-ui container

The nginx template in cameleer3-server-ui uses ${CAMELEER_API_URL} for
the upstream proxy target, not API_URL. The wrong env var name caused
the baked-in default (http://cameleer3-server:8081) to be used, which
doesn't resolve in per-tenant networks where the server is named
cameleer-server-{slug}.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -25,3 +25,4 @@ Thumbs.db
 # Generated by postinstall from @cameleer/design-system
 ui/public/favicon.svg
 docker/runtime-base/agent.jar
+.gitnexus