feat(ui): écran Bibliothèque d'exercices

Ajoute l'écran de bibliothèque d'exercices (presentation/exercise_library_screen.dart)
et ajuste la couche application/domaine (bootstrap, ports, use cases,
entités, repositories Drift) pour l'alimenter. flutter analyze propre,
9/9 tests verts, build APK debug validé.

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

lib/infrastructure/local/drift_repositories.dart

@@ -21,12 +21,28 @@ final class DriftExerciseRepository implements ExerciseRepository {
   Future<List<domain.Exercise>> listActive() async {
     final rows =
         await (database.select(database.exercises)
-              ..where((table) => table.deletedAt.isNull())
+              ..where(
+                (table) => table.deletedAt.isNull() & table.archivedAt.isNull(),
+              )
               ..orderBy([(table) => OrderingTerm.asc(table.name)]))
             .get();
     return rows.map(_exerciseFromRow).toList();
   }
 
+  @override
+  Future<bool> isReferencedByProgram(String id) async {
+    final row =
+        await (database.select(database.programExercises)
+              ..where(
+                (table) =>
+                    table.deletedAt.isNull() &
+                    table.sourceExerciseId.equals(id),
+              )
+              ..limit(1))
+            .getSingleOrNull();
+    return row != null;
+  }
+
   @override
   Future<void> save(domain.Exercise exercise) async {
     await database