Implement Contact Detail Service

Implement as a Riverpod AsyncNotifierProvider.family where the family parameter is the contactId. In the build method, use Future.wait([contactRepo.getContactDetail(id), assignmentRepo.getAssignment(id)]) for parallel fetching. Resolve field visibility rules via an injected OrgFieldVisibilityRules service (or a simple value class derived from the current user's org context read from a Riverpod auth provider). For the 30-second cache, store a _lastFetched timestamp in the notifier and check it at the start of build — if within the window, return the current state without re-fetching.

For prefetch on route push, use GoRouter's redirect or a route observer to call the notifier's prefetch method; alternatively, use Riverpod's ref.listen in the router configuration. Keep the ViewModel assembly in a private _buildViewModel() method for readability and independent testability.

Unit tests using flutter_test with mocked repository interfaces. Test cases: both repositories return successfully — verify ContactDetailViewModel assembled correctly, repository fetch runs in parallel (verify Future.wait used, not sequential await), contact-detail-repository fails — verify AsyncError emitted with correct failure type, assignment-repository fails — verify graceful degradation (contact data shown without assignment info, not total failure), contact not found — verify ContactNotFoundFailure distinguishable from NetworkFailure, NHF org context — verify chapter_affiliations included in ViewModel, Blindeforbundet context — verify sensitive_field_identifiers included. Cache test: two calls within 30s use same data (mock repository called only once). Target 90%+ coverage.