feat(sync): préparation à la synchronisation cloud (ticket #12)

Ajuste les ports et le bootstrap applicatif, ainsi que le mapping
Drift des repositories, pour préparer une future synchronisation cloud
(invariants documentés dans docs/sync-invariants.md). Corrige au
passage un bug de fuseau horaire UTC dans le mapping Drift. flutter
analyze propre, 35/35 tests verts, build APK debug validé.

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

lib/application/ports.dart

@@ -92,3 +92,26 @@ abstract interface class WorkoutHistoryRepository {
   Future<void> saveSetResult(WorkoutHistorySetResult result);
   Future<void> delete(String id, DateTime deletedAt);
 }
+
+final class SyncRunSummary {
+  const SyncRunSummary({
+    required this.pushedChanges,
+    required this.pulledChanges,
+  });
+
+  final int pushedChanges;
+  final int pulledChanges;
+}
+
+abstract interface class SyncGateway {
+  Future<SyncRunSummary> synchronize();
+}
+
+final class NoOpSyncGateway implements SyncGateway {
+  const NoOpSyncGateway();
+
+  @override
+  Future<SyncRunSummary> synchronize() async {
+    return const SyncRunSummary(pushedChanges: 0, pulledChanges: 0);
+  }
+}