feat(ui): navigation libre dans le plan de séance (ticket #22)

Permet de circuler librement entre les exercices/séries du plan de
séance sur workout_execution_screen.dart, au lieu d'un déroulé
strictement linéaire. flutter analyze propre, 31/31 tests verts, build
APK debug validé.

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

test/presentation/workout_execution_screen_test.dart

@@ -217,6 +217,69 @@ void main() {
 
     expect(repository.results.single.actualTimeMs, greaterThan(0));
   });
+
+  testWidgets('le plan de séance affiche les états des séries', (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: 2,
+      resolvedTemplateSnapshotJson: _sessionSnapshot(
+        setsCount: 4,
+        restSeconds: 0,
+      ),
+    );
+    repository.session = session;
+    repository.results.add(
+      ActiveSetResult(
+        metadata: _metadata('result-1'),
+        activeWorkoutSessionId: 'session-1',
+        programSnapshotId: 'template-program-1',
+        exerciseSnapshotId: 'exercise-1',
+        programIndex: 0,
+        exerciseIndex: 0,
+        setIndex: 0,
+        actualReps: 10,
+        actualScore: 7,
+        scoreUnitSnapshot: 'paniers',
+      ),
+    );
+
+    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('Voir le plan'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Plan de séance'), findsOneWidget);
+    expect(find.text('Terminée'), findsOneWidget);
+    expect(find.text('10 reps · 7 paniers'), findsOneWidget);
+    expect(find.text('Passée'), findsOneWidget);
+    expect(find.text('Aucun résultat'), findsOneWidget);
+    expect(find.text('En cours'), findsOneWidget);
+    expect(find.text('À faire'), findsOneWidget);
+
+    await tester.tap(find.text('En cours'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Plan de séance'), findsNothing);
+  });
 }
 
 ActiveWorkoutSessionUseCases _activeUseCases(