Headers and column widths

The header row is built from your column definitions:

• A string header is used as is.
• A function header cannot be written to a file, so set meta.rowpine.header; otherwise the column id is used.
• Columns inside header groups get the group names joined with /. In the image, the pivot's Units column under the Q1 group becomes Q1 / Units. One header row keeps the sheet easy to filter and sort.

Column widths come from TanStack's column sizes (size in the column definition, or the width after the user resizes). Set meta.rowpine.width (pixels) to give the file a different width than the screen.