feat: add booking edit modal for /today page with service management

Replace placeholder BookingModal on the Next Appointment edit button with a
dedicated EditBookingModal that allows admins to add, remove, and override
services on an active booking. Includes backend PUT endpoint with overlap
detection and full test suite (20 tests).

backend/main.go

@@ -236,6 +236,7 @@ func main() {
 				r.With(mw.RateLimit(60, time.Minute)).Get("/search", bookings.SearchAdminBookingsHandler)
 				r.Get("/user/{user_id}", bookings.GetAllBookingsByUserHandler)
 				r.Get("/{id}", bookings.GetAdminBookingHandler)
+				r.Put("/{id}", bookings.UpdateBookingServicesHandler)
 				r.Get("/{id}/overlapping", bookings.GetOverlappingBookingsHandler)
 				r.Put("/{id}/progress", bookings.ProgressBookingHandler)
 				r.Post("/{id}/confirm", bookings.ConfirmBookingHandler)