Configure Riverpod family provider and scoping

Use `final transcriptionProvider = StateNotifierProvider.autoDispose.family((ref, fieldId) { ... })`. AutoDispose is critical here — without it, providers for fields that have scrolled off-screen accumulate indefinitely. In the dispose body of TranscriptionStateManager, call `_speechService.cancel()` synchronously and `_partialRepo.persist(fieldId, state.partialTranscript)` as an unawaited Future (or await it in an override for tests).

Register `overrideWithValue` in ProviderScope for both SpeechRecognitionService and PartialTranscriptionRepository during widget and integration tests. Avoid placing the family declaration in the same file as TranscriptionStateManager to prevent circular imports — create a dedicated `transcription_providers.dart` in the component directory.

Unit tests (flutter_test + mocktail): (1) Verify two family instances with different fieldIds hold independent state after concurrent mutations. (2) Verify dispose triggers subscription cancellation on the mock SpeechRecognitionService. (3) Verify dispose calls PartialTranscriptionRepository.persist() with the correct in-flight transcript before stream is closed. (4) Verify ProviderScope.overrides correctly substitutes mock implementations for SpeechRecognitionService and PartialTranscriptionRepository.

Widget test: Mount a simple widget that watches two family providers simultaneously and confirm independent rebuild cycles. All tests must pass with zero Riverpod ProviderException warnings in output.