feat(exercice): icône d'exercice (ticket #38)

Étend le modèle Drift (tables.dart, app_database.dart/.g.dart), les
entités/use cases/repositories et les écrans bibliothèque d'exercices
et programme pour associer une icône à chaque exercice. flutter
analyze propre, 72/72 tests verts, build APK debug validé. Dernier
ticket du backlog.

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

lib/infrastructure/local/drift_repositories.dart

@@ -1078,6 +1078,7 @@ db.ExercisesCompanion _exerciseCompanion(domain.Exercise exercise) {
     remoteRevision: values[10] as Value<String?>,
     name: Value(exercise.name),
     description: Value(exercise.description),
+    iconMediaId: Value(exercise.iconMediaId),
     videoMediaId: Value(exercise.videoMediaId),
     hasTimeMeasure: Value(exercise.hasTimeMeasure),
     hasRepsMeasure: Value(exercise.hasRepsMeasure),
@@ -1099,6 +1100,7 @@ domain.Exercise _exerciseFromRow(db.Exercise row, List<String> imageMediaIds) {
     name: row.name,
     description: row.description,
     imageMediaIds: imageMediaIds,
+    iconMediaId: row.iconMediaId,
     videoMediaId: row.videoMediaId,
     hasTimeMeasure: row.hasTimeMeasure,
     hasRepsMeasure: row.hasRepsMeasure,