Implement PeerMentorCardWidget with mentor-specific data fields

Define an enum or sealed class for AvailabilityStatus (active, paused, unavailable) and CertificationStatus (current, expiring, expired) if not already present in the domain layer. Map each enum value to a design token color inside a helper method or extension, keeping color logic out of the widget's build method. For the certification 'expiring' state, compute whether a given expiry DateTime falls within 30 days of DateTime.now() in the data layer or a ViewModel — the widget itself should only receive the enum, not raw dates. Use a left-side colored border (Container with BoxDecoration borderRadius + border) or a subtly different card surface token to differentiate from ContactCardWidget without making the UI feel inconsistent.

Follow the same structural layout conventions as ContactCardWidget (padding, typography scale) to ensure visual coherence while still being distinguishable at a glance.

Write flutter_test widget tests covering: (1) rendering each availability status (active, paused, unavailable) and asserting the correct chip label and color token is applied; (2) rendering each certification status (current, expiring, expired) and asserting badge label and color; (3) zero assigned members shows '0 members' text without errors; (4) onTap callback fires when card is tapped; (5) visual distinction from ContactCardWidget — assert that the background color/decoration of PeerMentorCardWidget differs from ContactCardWidget when both are rendered in the same test tree. Use fixture data from task-001 mock data. No e2e tests required.