Build BLoC State Layer for Export Trigger Screen

Use flutter_bloc Cubit (not full Bloc) if event handling is straightforward — Cubit reduces boilerplate while still being testable with bloc_test. Define ExportTriggerState as a sealed class in Dart 3 using the sealed keyword; each subtype is a final class with only the fields it needs (e.g., ExportTriggerExporting only carries progress: int). Inject BufdirExportRepository as an abstract interface into the Cubit constructor — this makes unit testing trivial via fakes without needing http mocking. Wire the Riverpod provider using riverpod_annotation (@riverpod) for type-safe code generation, scoped to AutoDispose to prevent memory leaks.

The progress stream from the export API should be consumed with await for inside the Cubit's async method, emitting ExportTriggerExporting(progress: p) for each progress tick. Map all repository exceptions to user-friendly error strings in a private _mapError(Object e) method inside the Cubit — never pass raw exception messages to ExportFailedEvent.

Use the bloc_test package to write exhaustive state-transition tests: (1) happy path from idle → complete with progress updates at 0, 50, 100, (2) error path: ExportFailedEvent emits ExportTriggerError with sanitized message, (3) ConfirmCancelledEvent from confirming returns to formatSelected state with period and format preserved, (4) invalid transitions (e.g., PeriodSelectedEvent while exporting) do not change state, (5) exportingProgress is always 0–100 (test clamping at boundaries), (6) Riverpod provider auto-disposes when screen is popped (use ProviderContainer in test). Mock BufdirExportRepository using Mockito or manual fake. Target 100% line + branch coverage on Bloc logic.