fix(audio): ne coupe plus la musique externe sur les chronos de step (ticket #92)

Le son joué aux 3 dernières secondes d'un chrono de step n'interrompt plus la musique de l'utilisateur en cours de lecture.

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

lib/presentation/exercise_step_audio.dart

@@ -13,6 +13,9 @@ final class AudioplayersExerciseStepAudioCuePlayer
   AudioplayersExerciseStepAudioCuePlayer();
 
   final AudioPlayer _player = AudioPlayer();
+  late final Future<void> _audioContextReady = _player.setAudioContext(
+    _stepCueAudioContext,
+  );
 
   @override
   Future<void> playShortCountdownBeep() {
@@ -25,6 +28,7 @@ final class AudioplayersExerciseStepAudioCuePlayer
   }
 
   Future<void> _play(String assetPath) async {
+    await _audioContextReady;
     await _player.stop();
     await _player.play(AssetSource(assetPath));
   }
@@ -35,6 +39,18 @@ final class AudioplayersExerciseStepAudioCuePlayer
   }
 }
 
+final _stepCueAudioContext = AudioContext(
+  android: const AudioContextAndroid(
+    contentType: AndroidContentType.sonification,
+    usageType: AndroidUsageType.assistanceSonification,
+    audioFocus: AndroidAudioFocus.none,
+  ),
+  iOS: AudioContextIOS(
+    category: AVAudioSessionCategory.playback,
+    options: const {AVAudioSessionOptions.mixWithOthers},
+  ),
+);
+
 final class NoOpExerciseStepAudioCuePlayer
     implements ExerciseStepAudioCuePlayer {
   const NoOpExerciseStepAudioCuePlayer();