[genai-ui] Agent service dashboard: GenAI metrics subnav

Part of #381
Depends on: #389

## What

New subnav tab on the agent service dashboard showing GenAI metrics for that service.

- Timeseries charts: token usage over time (prompt/completion/total), request count, avg latency
- Per-model breakdown if the service calls multiple models
- Date range selector consistent with other dashboard views
- Data sourced from Scouter timeseries aggregate endpoint via OpsML proxy, scoped to service space/name/version
- Follows existing service dashboard layout and neo-brutalist chart patterns