Ship a default Grafana dashboard for Kubeflow Trainer

### What you would like to be added?

Ship a default Grafana dashboard as part of the Helm chart that provides out-of-the-box visibility
into controller health and TrainJob lifecycle.

This revives [#1376](https://github.com/kubeflow/trainer/issues/1376), filed in 2021 for V1 but
closed as stale.

### Challenges

- Dashboard depends on which metrics are available (controller-runtime defaults today, custom
metrics once the companion metrics issue lands)
- Needs to cover two personas: **platform operators** (controller health, reconcile backlogs)
and **ML engineers** (TrainJob lifecycle, time-to-running, failure rates)
- Should be gated in Helm values to avoid installing for clusters without Grafana

### Why is this needed?

### Current State

- No dashboard shipped with Trainer
- Operators must manually discover available metrics, write PromQL, and build panels
- Peer projects (Kueue, Argo Workflows) bundle dashboards - Trainer is a gap

### Related Issues

- https://github.com/kubeflow/trainer/issues/1376
- https://github.com/kubeflow/trainer/issues/1340
- https://github.com/kubeflow/trainer/issues/2648
- https://github.com/kubeflow/trainer/issues/2779

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