Implement BufdirPreviewScreen scaffold and layout
epic-bufdir-report-preview-screen-and-diff-task-001 — Create the top-level BufdirPreviewScreen StatefulWidget with full-screen layout, AppBar with header actions (export button, diff toggle), and placeholder slots for validation summary banner, scrollable report section list, and footer navigation. Wire up BufdirPreviewService via Riverpod provider and handle loading/error states.
Acceptance Criteria
Technical Requirements
Implementation Notes
Extend ConsumerStatefulWidget (not StatefulWidget) so ref is available in State methods without passing it around. Use AsyncValue.when() for the three UI states to keep build() clean. Define the AppBar actions as private _buildActions() method returning a List
The footer navigation should reuse the shared bottom nav widget from the design system — do not create a new one. Place placeholder Container widgets with a fixed height (0) for the banner and section list slots so the layout contract is established before child tasks fill them in. Use GoRouter's extra parameter (typed) to pass the session/report context from the calling screen rather than fetching again.
Testing Requirements
Write widget tests using flutter_test. Test (1) loading state renders CircularProgressIndicator, (2) error state renders error message and retry button, (3) loaded state renders AppBar with both action buttons, (4) provider is called exactly once on first build, (5) route receives reportId parameter correctly. Mock BufdirPreviewService with a ProviderScope override returning AsyncLoading, AsyncError, and AsyncData variants. Aim for 100% branch coverage of the loading/error/data conditional.
Run flutter analyze with zero warnings before merging.
The preview screen must pass period, scope, and aggregation session state to the Bufdir Report Export screen via navigation arguments. If the export screen's navigation argument schema is not yet finalized when this epic begins, the handoff will require rework — potentially after TestFlight is already in use by coordinators.
Mitigation & Contingency
Mitigation: Define the shared BufdirExportNavigationArgs Dart class jointly with the Bufdir Report Export feature team before this epic starts. Store it in a shared models package both features depend on. Treat the class as an API contract with a minor-version bump policy.
Contingency: If the export screen's argument schema changes after the preview screen is implemented, the BufdirPreviewService session state model can be adapted with a compatibility shim. The preview screen itself requires only a one-line navigation call change.
The period diff view loads prior-period data on demand and renders it inline with the current report. On a large report (many sections, many fields) combined with slow Supabase connectivity, the diff overlay could block the UI or produce a janky re-render that degrades the coordinator experience during the pre-submission review.
Mitigation & Contingency
Mitigation: Load prior-period data in a background Riverpod FutureProvider that starts prefetching when the preview screen mounts (not when the user taps the diff toggle). Show a shimmer placeholder on each field row's prior-period column while loading. Cache prior-period data in BufdirPreviewRepository using the same local cache as current-period data.
Contingency: If diff view performance is unacceptable on TestFlight devices, disable the toggle for the initial TestFlight release and ship the diff view in the following sprint after profiling the Supabase query plan and adding an appropriate Postgres index on the prior-period data table.
The full preview screen will be reviewed by coordinators on TestFlight who will cross-reference it against their physical Bufdir reporting forms. Any section ordering difference or label mismatch discovered during UAT will require a fix before the feature can be signed off, potentially delaying the entire Bufdir reporting pipeline.
Mitigation & Contingency
Mitigation: Conduct a pre-TestFlight alignment review with at least one coordinator from NHF and one from HLF using a static screenshot of the preview screen layout. Obtain written sign-off on section order and field labels before distributing to the full test group of 5-8 people.
Contingency: If label mismatches are found during TestFlight UAT, update BufdirReportStructureMapper constants and rebuild. Since the mapper is a pure Dart class with no persisted state, corrections are deployed in the next TestFlight build with no database migration required.