### What
This PR updates the import-export plugin's `<Preview />` component to
render table columns and rows using the same logic as the CSV export.
Key changes:
- Adds a new `/api/preview-data` custom REST endpoint that:
- Accepts filters (`fields`, `where`, `sort`, `draft`, `limit`)
- Uses `getCustomFieldFunctions` and `flattenObject` to transform
documents
- Returns deeply flattened rows identical to the CSV export
- Refactors the <Preview /> component to:
- POST preview config to the new endpoint instead of querying the
collection directly
- Match column ordering and flattening logic with the `createExport`
function
- Ensures consistency across CSV downloads and in-admin previews
-Adds JSON preview
This ensures preview results now exactly match exported CSV content,
including support for custom field transformers and polymorphic fields.
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Co-authored-by: Dan Ribbens <dan.ribbens@gmail.com>
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