Assemble Contact Detail Screen

Use a `StreamProvider` (Riverpod) that wraps `contact-detail-service.watchContact(id)` to reactively drive the screen. The screen widget is a `ConsumerWidget` that maps the `AsyncValue` to loading/error/data states using `.when()`. Pre-warm the contact data by passing the contact snapshot from the contacts list navigation into the Riverpod provider family as an initial value — this achieves the 500ms target without waiting for the Supabase round-trip. Use a `CustomScrollView` with `SliverToBoxAdapter` sections to allow smooth scroll performance across heterogeneous content.

Organisation-specific sections (`AssignmentStatusIndicator`, `MultiChapterAffiliationChip`, encrypted fields) should be conditionally rendered based on `contact.organisationType` and `contact.roles` — centralise this logic in a `ContactDetailSectionResolver` helper to keep the screen widget clean. For accessibility traversal, wrap the profile header, each section, and the activity list in `MergeSemantics` or explicit `Semantics` nodes with `sortKey` to enforce VoiceOver reading order independent of widget tree order.

Widget tests (flutter_test): (1) render with NHF coordinator contact — verify header, chapter chips, activity list, no encrypted fields, no assignment indicator; (2) render with Blindeforbundet contact — verify encrypted field widgets present, assignment indicator visible, no chapter chips; (3) render with peer mentor contact — verify peer mentor button/tab is present; (4) render with non-peer-mentor — verify peer mentor button absent; (5) render as read-only role — verify edit button absent; (6) render as coordinator — verify edit button present and tappable; (7) loading state — verify skeleton renders; (8) error state — verify retry button; (9) contact not found — verify 404 state. Integration test: navigate via GoRouter to `contact/:id` with a mock route parameter and verify correct contact is loaded. Accessibility: full `SemanticsHandle` traversal test — verify all interactive elements have labels and traversal order. Performance: measure frame build time for initial render using `WidgetTester.pump()` timing.

Target ≥85% widget test coverage.