fix(execution): comportement de la flèche retour en séance (ticket #31)

Corrige la flèche de retour sur workout_execution_screen.dart : elle
ouvre désormais le choix pause/abandon ou quitter-et-sauvegarder au
lieu de ne rien faire. flutter analyze propre, 36/36 tests verts,
build APK debug validé.

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

lib/presentation/workout_execution_screen.dart

@@ -78,8 +78,17 @@ final class _WorkoutExecutionScreenState extends State<WorkoutExecutionScreen> {
   Widget build(BuildContext context) {
     return PopScope(
       canPop: false,
+      onPopInvoked: _handleSystemBack,
       child: Scaffold(
         appBar: AppBar(
+          automaticallyImplyLeading: false,
+          leading: _canPauseFromNavigation
+              ? IconButton(
+                  tooltip: 'Mettre en pause',
+                  onPressed: () => unawaited(_pause()),
+                  icon: const Icon(Icons.arrow_back),
+                )
+              : null,
           title: Text(_plan.name),
           actions: [
             IconButton(
@@ -263,6 +272,16 @@ final class _WorkoutExecutionScreenState extends State<WorkoutExecutionScreen> {
     );
   }
 
+  bool get _canPauseFromNavigation {
+    return _mode == WorkoutExecutionMode.active ||
+        _mode == WorkoutExecutionMode.rest;
+  }
+
+  void _handleSystemBack(bool didPop) {
+    if (didPop || !_canPauseFromNavigation) return;
+    unawaited(_pause());
+  }
+
   Future<void> _pause() async {
     _restTicker?.cancel();
     _session = await widget.activeUseCases.pause(_session.metadata.id);