Bug fix report for sprint 636 (fixes deployed on August 6)

Issue 1:
Excel export button missing from Admin > Billing > Invoiced

Bug description:
The Excel export button was not visible in the grid in Admin > Billing > Invoiced.

Resolution:
The button is now visible again.

---

Issue 2:
Cost center requirement not satisfied despite pre-filled value

Bug description:
When a cost center was required for a booking and automatically filled via a group, request, or project, the system did not recognize the requirement as fulfilled. Users had to manually reselect the cost center from the dropdown to proceed.

Resolution:
The system now correctly recognizes pre-filled cost centers (e.g., through group selection) and no longer requires manual confirmation.

---

Issue 3:
Cost center information lost when updating a booking

Bug description:
When an existing booking was updated, any associated cost center information was lost.

Resolution:
Cost center data is now retained when a booking is updated.