feat(gestion): suppression en cascade exercice/programme/séance-modèle (ticket #37)

Ajoute la suppression avec gestion des dépendances en cascade sur les
écrans bibliothèque d'exercices, programme et séance-modèle, avec les
use cases et le bootstrap applicatif associés. flutter analyze propre,
58/58 tests verts, build APK debug validé.

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

lib/presentation/exercise_library_screen.dart

@@ -157,6 +157,7 @@ final class _ExerciseLibraryScreenState extends State<ExerciseLibraryScreen> {
                       return ExerciseListTile(
                         exercise: exercise,
                         onTap: () => _openForm(exercise: exercise),
+                        onDelete: () => _confirmDelete(exercise),
                       );
                     }, childCount: filtered.length * 2 - 1),
                   ),
@@ -203,13 +204,64 @@ final class _ExerciseLibraryScreenState extends State<ExerciseLibraryScreen> {
       _reload();
     }
   }
+
+  Future<void> _confirmDelete(Exercise exercise) async {
+    final used = await widget.exerciseUseCases.isUsedInPrograms(
+      exercise.metadata.id,
+    );
+    if (!mounted) return;
+    final confirmed = await showDialog<bool>(
+      context: context,
+      builder: (context) => AlertDialog(
+        title: Text('Supprimer ${exercise.name} ?'),
+        content: Text(
+          used
+              ? 'Cet exercice est utilisé dans un ou plusieurs programmes. '
+                    'Le supprimer le retirera de ces programmes. Continuer ?'
+              : 'Cette action est irréversible.',
+        ),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.of(context).pop(false),
+            child: const Text('Annuler'),
+          ),
+          FilledButton.icon(
+            onPressed: () => Navigator.of(context).pop(true),
+            icon: const Icon(Icons.delete_outline),
+            label: const Text('Supprimer'),
+          ),
+        ],
+      ),
+    );
+    if (confirmed != true) return;
+    try {
+      await widget.exerciseUseCases.delete(exercise.metadata.id);
+      if (!mounted) return;
+      _reload();
+    } on Exception catch (error) {
+      _showSnackBar(error.toString());
+    }
+  }
+
+  void _showSnackBar(String message) {
+    if (!mounted) return;
+    ScaffoldMessenger.of(
+      context,
+    ).showSnackBar(SnackBar(content: Text(message)));
+  }
 }
 
 final class ExerciseListTile extends StatelessWidget {
-  const ExerciseListTile({required this.exercise, this.onTap, super.key});
+  const ExerciseListTile({
+    required this.exercise,
+    this.onTap,
+    this.onDelete,
+    super.key,
+  });
 
   final Exercise exercise;
   final VoidCallback? onTap;
+  final VoidCallback? onDelete;
 
   @override
   Widget build(BuildContext context) {
@@ -258,7 +310,19 @@ final class ExerciseListTile extends StatelessWidget {
           ),
         ],
       ),
-      trailing: const Icon(Icons.chevron_right),
+      trailing: onDelete == null
+          ? const Icon(Icons.chevron_right)
+          : Row(
+              mainAxisSize: MainAxisSize.min,
+              children: [
+                IconButton(
+                  tooltip: 'Supprimer l’exercice',
+                  icon: const Icon(Icons.delete_outline),
+                  onPressed: onDelete,
+                ),
+                const Icon(Icons.chevron_right),
+              ],
+            ),
     );
   }
 }