feat: add server-side API proxying via nginx

[joshuasequeira19]

Previously, the React app tried to call the API server directly from the browser using cluster-internal DNS (forkspacer-api-server:8080), which fails as browsers cannot resolve Kubernetes service DNS names.

This change implements server-side API proxying:

• Browser calls relative paths (/api/v1/*)
• Nginx in operator-ui pod proxies to api-server internally
• API requests never leave the cluster from the browser

Changes:

• Add nginx.conf.template with proxy configuration
• Add docker-entrypoint.sh to inject API_SERVER_URL at runtime
• Update Dockerfile to use custom nginx config and entrypoint
• Update Helm values to use API_SERVER_URL for nginx proxying
• Update deployment template to pass API_SERVER_URL to container

[github-actions]

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