### What?
This PR adds a dedicated `sortOrder` select field (Ascending /
Descending) to the import-export plugin, alongside updates to the
existing `SortBy` component. The new field and component logic keep the
sort field (`sort`) in sync with the selected sort direction.
### Why?
Previously, descending sorting did not work. While the `SortBy` field
could read the list view’s `query.sort` param, if the value contained a
leading dash (e.g. `-title`), it would not be handled correctly. Only
ascending sorts such as `sort=title` worked, and the preview table would
not reflect a descending order.
### How?
- Added a new `sortOrder` select field to the export options schema.
- Implemented a `SortOrder` custom component using ReactSelect:
- On new exports, reads `query.sort` from the list view and sets both
`sortOrder` and `sort` (combined value with or without a leading dash).
- Handles external changes to `sort` that include a leading dash.
- Updated `SortBy`:
- No longer writes to `sort` during initial hydration—`SortOrder` owns
initial value setting.
- On user field changes, writes the combined value using the current
`sortOrder`.
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