Timeline details: show definition contents, not just ids #167

The Timeline details panel shows definition ids but never what they mean. The data is already in the graph payload (metadata.schemas, metadata.partition-specs, metadata.sort-orders — Iceberg keeps these arrays append-only, so every referenced version is present).

Three gaps, in value order:

1. Schema contents / diff. The panel shows Schema ID 2 and rows say schema changed (v1 → v2), but never the columns. A schema-changed commit should show the field-level diff (e.g. added column email: string), and the Metadata file section could expand the current schema's column list on demand.
2. Partition spec contents. Partition Spec ID 0 should be expandable to its transforms (e.g. bucket(4, id), days(ts)); partitioning-changed commits should show old → new fields.
3. Sort order contents. Same shape: transforms + direction/null-order per field.

Needs a design decision on where the expanded definitions live (inline in the Metadata file section vs. a popover vs. only on change-commits), which is why this is split out of the core Timeline page work.

Frontend-only; no API changes required.