Unit test repositories and dispatcher

Use mocktail (preferred over mockito for null-safety ergonomics) to create mock classes for SupabaseClient and its query chain. For the Supabase query builder chain (from().select().eq().single()), use a fake implementation that returns pre-configured responses — the fluent builder pattern makes deep mocking brittle, so prefer a thin wrapper interface on each repository that can be cleanly mocked. For the offline/cache fallback test, throw a SocketException from the mocked client and assert the repository returns the previously cached value. For the RLS test, return a PostgrestException with code '42501' and assert the repository maps it to RepositoryException(code: cross_chapter_access_denied).

Group tests by repository class for readability and to allow selective execution with flutter test --name.

This task IS the testing task. Organise tests into three describe groups: ScenarioRuleRepository, PromptHistoryRepository, PushNotificationDispatcher. Use mocktail to mock SupabaseClient, SupabaseQueryBuilder, and the Edge Function invoker. Each group must cover the happy path, the main failure path, and at least one edge case (cache, deduplication, offline).

Use setUp/tearDown to reset mock state. Assert both return values and side effects (e.g., delete called on mocked client). Run with flutter test --coverage and verify ScenarioRuleRepository, PromptHistoryRepository, and PushNotificationDispatcher each exceed 80% line coverage.