Admin UI: add a direct link to the Grafana metrics dashboard #779

Context

Operators use the Admin UI to inspect OpenRAG and Grafana to investigate runtime metrics.

Problem

There is no direct path between these tools, so operators must know and navigate to the Grafana address separately.

Expected behavior

• The Admin UI provides a clear metrics action that opens the configured Grafana dashboard.
• Deployments can configure the destination without changing the UI.
• The action is hidden or disabled when no dashboard is configured.