[Groups] Add users to group (POST /groups-management/groups/{id}/users)

[TheRealSeber]

Implement remote function and UI for adding users to a group via POST /groups-management/groups/{id}/users.

Requirements

• Design add-users workflow (e.g. modal, dialog, select dropdown)
• Connect to backend via SvelteKit remote function: POST /groups-management/groups/{id}/users
• Input validation and UX feedback (valibot, notifications)
• Match Tasks management add-user UI flows
• Add full i18n (Paraglide)

Child of [Groups] Implement groups management for teachers (parent issue): #139 READ IT before implementing anything