feat(exercice): affichage des médias pendant l'exécution (ticket #36)

Expose la galerie d'images d'exercice (backend du ticket #35) pendant
l'exécution de séance sur workout_execution_screen.dart et
program_screen.dart, avec les ajustements domain/use cases/repositories
nécessaires. flutter analyze propre, 67/67 tests verts, build APK
debug validé.

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

test/application/use_cases_test.dart

@@ -225,6 +225,66 @@ void main() {
     expect(updated.imageMediaId, 'media-1');
   });
 
+  test('program exercise snapshot keeps the full image gallery', () async {
+    final exerciseRepository = _FakeExerciseRepository()
+      ..exercise = Exercise(
+        metadata: _metadata('exercise-1'),
+        name: 'Squat',
+        imageMediaIds: const ['media-1', 'media-2', 'media-3'],
+        videoMediaId: 'video-1',
+        hasTimeMeasure: false,
+        hasRepsMeasure: true,
+        hasScoreMeasure: false,
+        defaultTargetReps: 10,
+      );
+    final programRepository = _FakeProgramRepository()
+      ..programs.add(
+        Program(
+          metadata: _metadata('program-1'),
+          name: 'Jambes',
+          defaultRestSeconds: 60,
+        ),
+      );
+    final useCase = ProgramUseCases(
+      programRepository: programRepository,
+      exerciseRepository: exerciseRepository,
+      templateRepository: _FakeWorkoutTemplateRepository(),
+      clock: _FakeClock(DateTime.utc(2026, 7, 17, 12)),
+      ids: _FakeIds(),
+      originDeviceId: 'device-1',
+    );
+
+    final snapshot = await useCase.addExercise(
+      programId: 'program-1',
+      exerciseId: 'exercise-1',
+      position: 0,
+      setsCount: 3,
+      enabledMeasures: const {WorkoutMeasure.reps},
+    );
+
+    expect(snapshot.exerciseImageMediaIdSnapshot, 'media-1');
+    expect(snapshot.exerciseImageMediaIdsSnapshot, [
+      'media-1',
+      'media-2',
+      'media-3',
+    ]);
+    expect(snapshot.exerciseVideoMediaIdSnapshot, 'video-1');
+    expect(snapshot.toSnapshotJson()['imageMediaIdsSnapshot'], [
+      'media-1',
+      'media-2',
+      'media-3',
+    ]);
+    expect(
+      programRepository
+          .programs
+          .single
+          .exercises
+          .single
+          .exerciseImageMediaIdsSnapshot,
+      snapshot.exerciseImageMediaIdsSnapshot,
+    );
+  });
+
   test('delete exercise removes it from referencing programs', () async {
     final exerciseRepository = _FakeExerciseRepository()
       ..exercise = Exercise(