Integrate broker-monitor features into the GUI

## Motivation

[aktin/broker-monitor](https://github.com/aktin/broker-monitor) provides node monitoring on top of the broker API. Its features should become part of this GUI so no separate tooling is needed.

## Candidate features (scope to be refined)

- Node connectivity overview: online/offline state derived from last-contact timestamps, with configurable thresholds
- Import statistics per node: imported/updated/invalid/failed counts and error rate, including trend over time
- Highlighting of nodes with stale contact or rising error rates

No files or tests are named. Start by reviewing aktin/broker-monitor and the broker API to refine the feature scope; done means the agreed node connectivity, import statistics and trends, and stale/error highlighting are integrated into this GUI without separate tooling.

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