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);

test/presentation/workout_execution_screen_test.dart

@@ -218,6 +218,47 @@ void main() {
     expect(repository.results.single.actualTimeMs, greaterThan(0));
   });
 
+  testWidgets('la flèche de retour met la séance active en pause', (
+    tester,
+  ) async {
+    final clock = _FakeClock(DateTime.utc(2026, 7, 17, 12));
+    final repository = _FakeActiveSessionRepository();
+    final session = ActiveWorkoutSession(
+      metadata: _metadata('session-1'),
+      sourceWorkoutTemplateId: 'template-1',
+      status: ActiveWorkoutStatus.running,
+      startedAt: DateTime.utc(2026, 7, 17, 12),
+      lastPersistedAt: DateTime.utc(2026, 7, 17, 12),
+      elapsedActiveMs: 0,
+      currentProgramIndex: 0,
+      currentExerciseIndex: 0,
+      currentSetIndex: 0,
+      resolvedTemplateSnapshotJson: _sessionSnapshot(
+        setsCount: 2,
+        restSeconds: 0,
+      ),
+    );
+    repository.session = session;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: WorkoutExecutionScreen(
+          initialSession: session,
+          activeUseCases: _activeUseCases(repository, clock),
+          closeUseCase: _closeUseCase(repository, clock),
+          historyUseCases: _historyUseCases(clock),
+          workoutTemplateUseCases: _workoutTemplateUseCases(),
+        ),
+      ),
+    );
+
+    await tester.tap(find.byTooltip('Mettre en pause'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Reprendre'), findsOneWidget);
+    expect(repository.session?.status, ActiveWorkoutStatus.paused);
+  });
+
   testWidgets('le plan de séance affiche les états des séries', (tester) async {
     final clock = _FakeClock(DateTime.utc(2026, 7, 17, 12));
     final repository = _FakeActiveSessionRepository();