As a user I would like to export cost data

Note:
This item should be part of GA

• What: CSV/JSON export of the current table view (filters, sort applied); async download with feedback.

• Where:

• FE: src/components/cost-table/ExportButton.tsx

• BE (optional for server-side export): router.ts → /export endpoint

• Notes: include i18n-friendly headers; large dataset handling.