Allow creation of multi-site groups to combine Perinatal and Pediatric activities

Affected users: Administrative staff and healthcare providers/educators managing multi-site group activities in Fondation du Dr Julien.
Task affected: Group creation and management process within the MYLE administration/group module, specifically when setting up joint activities involving participants from different sites (e.g., Perinatal and Pediatric).
Impact: Occurs whenever mixed-site groups are created (e.g., weekly recurring activities like Musical Awakening on Fridays). Creating duplicate groups creates extra administrative overhead, increases management friction, and adds unnecessary steps to maintain accurate statistics and reporting for each site.
Current state: Administrative staff must manually create the same group twice with slight name variations (one per site) to assign children according to their respective sites. This workaround preserves statistical accuracy and reporting integrity, but requires duplicate management.
Competition consideration: N/A