Retrieves and displays milestone information for a specific user within a referral program. If programId is not provided, it defaults to fetching the first active program associated with the brand. If a milestone has been achieved, it includes details such as the milestone name, number of referrals required, time period in days, status as 'completed', and any associated reward item ID. If a milestone is pending, it includes the remaining referrals and days needed to achieve it and sets the status accordingly. If no milestones are configured for the program, it returns an error message indicating the absence of milestones.