Make clickable data-table rows keyboard accessible

## Summary

The shared `DataTable` component makes rows clickable when an `onRowClick` handler is supplied. Those rows are mouse-accessible, but they cannot currently receive keyboard focus or be opened with the keyboard.

The component is used by the evaluation-build and evaluation-run screens, so a focused fix improves both locations.

## Scope

- Make only interactive data-table rows keyboard focusable.
- Support Enter and Space to invoke the same callback as a row click.
- Preserve the current behavior that clicks on nested buttons and form controls do not trigger the row action.
- Keep non-interactive rows unchanged.

## Acceptance criteria

- A keyboard user can tab to an interactive row.
- Pressing Enter or Space opens/activates the same row action as a mouse click.
- Nested buttons, inputs, selects, and textareas remain independently usable.
- The frontend build and lint checks pass.

## Helpful files

- `frontend/src/components/ui/data-table.tsx`
- `frontend/src/features/evaluation-builds/page.tsx`
- `frontend/src/features/evaluations/page.tsx`

Please keep this focused on keyboard interaction; a broader table-component rewrite is out of scope.