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>

lib/application/ports.dart

@@ -80,6 +80,7 @@ abstract interface class ActiveSessionRepository {
   Future<void> save(ActiveWorkoutSession session);
   Future<void> saveSetResult(ActiveSetResult result);
   Future<void> saveRestState(ActiveRestState restState);
+  Future<ActiveRestState?> findRestStateById(String id);
   Future<List<ActiveSetResult>> listSetResults(String sessionId);
   Future<List<ActiveRestState>> listRestStates(String sessionId);
 }