Improve trace viewer for large trace logs

[edbeeching]

Summary

• Reworks the Traces tab into a dataset-viewer-style table with page-sized loading, run selection, URL state, column toggles, search, sorting, and inline expandable cells.
• Adds a step slider/count view so large batched trace runs can load one training step at a time.
• Uses scalar-only log loading for the Metrics tab so large trace payloads are not fetched just to render scalar charts.
• Loads full-resolution scalar metric rows for Metrics so smoothing is computed over all scalar points rather than a sampled 1500-point subset.

Notes

• Known gap: multimodal trace message parts/images are not preserved as inline media in this draft table view; non-string message content is rendered as JSON.

Validation

• npm --prefix trackio/frontend run lint
• npm --prefix trackio/frontend run build
• npm --prefix trackio/frontend run test
• Local trackio show smoke test against project trackio-long-trace-metrics-repro