Add SupersededAtUtc to ArrangementPolicy; hide superseded types from New Arrangement UI

[Copilot]

ArrangementPolicy lacked a way to mark arrangement types as superseded, causing outdated types to appear in the "+ New Arrangement" button list on the client family screen.

Changes

• IPoliciesResource.cs — Added DateTime? SupersededAtUtc = null to ArrangementPolicy, matching the existing pattern on VolunteerRolePolicyVersion / VolunteerFamilyRolePolicyVersion
• GeneratedClient.ts — Added supersededAtUtc to ArrangementPolicy class and IArrangementPolicy interface with standard date serialization/deserialization
• ArrangementsSection.tsx — Filter out superseded policies before rendering the create-arrangement buttons:

  policy.referralPolicy?.arrangementPolicies
    ?.filter((arrangementPolicy) => !arrangementPolicy.supersededAtUtc)
    .map(...)

  Existing arrangements of superseded types continue to display normally in arrangement cards.