feat(ui): écran Exécution de séance

Ajoute l'écran d'exécution de séance (presentation/workout_execution_screen.dart),
avec les ajustements application/domaine/repositories nécessaires et la
navigation depuis l'accueil et l'écran séance-modèle. flutter analyze
propre, 15/15 tests verts, build APK debug validé.

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

lib/presentation/workout_template_screen.dart

@@ -4,16 +4,21 @@ import 'package:flutter/material.dart';
 
 import '../application/application.dart';
 import '../domain/domain.dart';
+import 'workout_execution_screen.dart';
 
 final class WorkoutTemplateListScreen extends StatefulWidget {
   const WorkoutTemplateListScreen({
     required this.workoutTemplateUseCases,
     required this.programUseCases,
+    required this.activeUseCases,
+    required this.closeUseCase,
     super.key,
   });
 
   final WorkoutTemplateUseCases workoutTemplateUseCases;
   final ProgramUseCases programUseCases;
+  final ActiveWorkoutSessionUseCases activeUseCases;
+  final CloseWorkoutSessionUseCase closeUseCase;
 
   @override
   State<WorkoutTemplateListScreen> createState() =>
@@ -65,7 +70,10 @@ final class _WorkoutTemplateListScreenState
                 trailing: Wrap(
                   spacing: 8,
                   children: [
-                    TextButton(onPressed: () {}, child: const Text('Lancer')),
+                    TextButton(
+                      onPressed: () => _start(template),
+                      child: const Text('Lancer'),
+                    ),
                     const Icon(Icons.chevron_right),
                   ],
                 ),
@@ -99,6 +107,22 @@ final class _WorkoutTemplateListScreenState
       _reload();
     }
   }
+
+  Future<void> _start(WorkoutTemplate template) async {
+    final session = await widget.activeUseCases.startFromTemplate(
+      template.metadata.id,
+    );
+    if (!mounted) return;
+    await Navigator.of(context).push(
+      MaterialPageRoute(
+        builder: (context) => WorkoutExecutionScreen(
+          initialSession: session,
+          activeUseCases: widget.activeUseCases,
+          closeUseCase: widget.closeUseCase,
+        ),
+      ),
+    );
+  }
 }
 
 final class WorkoutTemplateFormScreen extends StatefulWidget {