Enable donors to view and change their subscription selections in the email preference centre
NOTE: Related/adjacent, not duplicated: double-opt-in display and cancellation are handled in T417935 and T416702; SMS-specific opt-out in the same preferences UI is T390757 (sibling requirement so whoever implements this should coordinate so the two opt-out mechanisms share a coherent UI)
Donor can see their current subscription selections in the preference centre.
Donor can add or remove subscriptions and save changes, with changes written to CiviCRM as the source of record.
Where a selection requires double opt-in, the flow defers to the DOI handling in T417935 rather than reimplementing it.
Changes are reflected in the status indicator (T427288) without a separate refresh.