text: Title: Improve "Link existing booking" feature
  Body: When you click on "Link existing booking", you are redirected to the bookings page:
  ![image](https://github.com/user-attachments/assets/86e072ac-ac22-4d91-8cca-ffd40481bda2)
  
  There are two things we could improve here:
  - Currently, the bookings page uses the current date. It'd be better if it used the date of the event (or the first day in case of multi-day events)
  - The bookings page currently shows all bookings which makes it hard to find a booking which can be linked. We should filter the bookings using the linked event to only show relevant bookings.
  
  cc @Moliholy @OmeGak 