fix(persistence): correctifs post-QA sur les repositories (ticket #13)

Corrige les ports, use cases, entités et l'implémentation Drift des
repositories suite aux retours de QA. flutter analyze propre, 24/24
tests verts, build APK debug validé.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/presentation/workout_execution_screen_test.dart

@@ -230,6 +230,9 @@ final class _FakeActiveSessionRepository implements ActiveSessionRepository {
   @override
   Future<ActiveWorkoutSession?> findOpen() async => session;
 
+  @override
+  Future<ActiveRestState?> findRestStateById(String id) async => null;
+
   @override
   Future<List<ActiveRestState>> listRestStates(String sessionId) async {
     return const [];