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/application/use_cases.dart

@@ -46,6 +46,44 @@ final class ExerciseUseCases {
 
   Future<void> save(Exercise exercise) => repository.save(exercise);
 
+  Future<List<Exercise>> listActive() => repository.listActive();
+
+  Future<bool> isUsedInPrograms(String id) {
+    return repository.isReferencedByProgram(id);
+  }
+
+  Future<Exercise> update({
+    required String id,
+    required String name,
+    String? description,
+    String? imageMediaId,
+    String? videoMediaId,
+    required bool hasTimeMeasure,
+    required bool hasRepsMeasure,
+    required bool hasScoreMeasure,
+    String? scoreLabel,
+    String? scoreUnit,
+  }) async {
+    final exercise = await repository.findById(id);
+    if (exercise == null) {
+      throw const DomainException('Exercise not found.');
+    }
+    final updated = exercise.copyWith(
+      metadata: exercise.metadata.touch(clock.now()),
+      name: name,
+      description: description,
+      imageMediaId: imageMediaId,
+      videoMediaId: videoMediaId,
+      hasTimeMeasure: hasTimeMeasure,
+      hasRepsMeasure: hasRepsMeasure,
+      hasScoreMeasure: hasScoreMeasure,
+      scoreLabel: scoreLabel,
+      scoreUnit: scoreUnit,
+    );
+    await repository.save(updated);
+    return updated;
+  }
+
   Future<Exercise> attachImage({
     required String exerciseId,
     required String mediaAssetId,