fix(exercice): score par défaut à 0 et affichage des répétitions en séance (tickets #44, #45)

Autorise un score par défaut à 0 (#44) et corrige l'affichage des
répétitions à 0 pendant l'exécution au lieu de la vraie cible (#45),
sur exercise_library_screen.dart et workout_execution_screen.dart,
avec les ajustements domain/use cases associés. flutter analyze
propre (mêmes 8 infos préexistantes), 79/79 tests verts, build APK
debug validé.

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

lib/application/use_cases.dart

@@ -1646,8 +1646,8 @@ void _validateExerciseDefaultTargets({
   }
   if (hasScoreMeasure &&
       scoreInputMode == ScoreInputMode.manual &&
-      (defaultTargetScore == null || defaultTargetScore <= 0)) {
-    throw const DomainException('Default target score must be positive.');
+      (defaultTargetScore == null || defaultTargetScore < 0)) {
+    throw const DomainException('Default target score must not be negative.');
   }
   if (defaultTargetScore != null &&
       (!hasScoreMeasure || scoreInputMode != ScoreInputMode.manual)) {

lib/domain/entities.dart

@@ -167,7 +167,10 @@ final class Exercise {
       'Default target time seconds',
     );
     _requireNullablePositive(defaultTargetReps, 'Default target reps');
-    _requireNullablePositiveDouble(defaultTargetScore, 'Default target score');
+    _requireNullableNonNegativeDouble(
+      defaultTargetScore,
+      'Default target score',
+    );
     _requireNullablePositive(
       defaultTargetScoreTimeMs,
       'Default target score time ms',
@@ -1035,12 +1038,6 @@ void _requireNullableNonNegativeDouble(double? value, String label) {
   }
 }
 
-void _requireNullablePositiveDouble(double? value, String label) {
-  if (value != null && value <= 0) {
-    throw DomainException('$label must be positive.');
-  }
-}
-
 void _requireScoreTargetShape({
   required bool scoreEnabled,
   required ScoreInputMode scoreInputMode,

lib/presentation/exercise_library_screen.dart

@@ -635,9 +635,9 @@ final class _ExerciseFormScreenState extends State<ExerciseFormScreen> {
                         _scoreInputMode != ScoreInputMode.manual) {
                       return null;
                     }
-                    return _positiveDoubleValidator(
+                    return _nonNegativeDoubleValidator(
                       value,
-                      'Saisis un score supérieur à 0.',
+                      'Saisis un score positif ou nul.',
                     );
                   },
                 ),
@@ -914,6 +914,14 @@ final class _ExerciseFormScreenState extends State<ExerciseFormScreen> {
     return null;
   }
 
+  String? _nonNegativeDoubleValidator(String? value, String message) {
+    final number = double.tryParse(value?.trim() ?? '');
+    if (number == null || number < 0) {
+      return message;
+    }
+    return null;
+  }
+
   String _optionalIntText(int? value) {
     return value == null ? '' : '$value';
   }

lib/presentation/workout_execution_screen.dart

@@ -72,6 +72,7 @@ final class _WorkoutExecutionScreenState extends State<WorkoutExecutionScreen> {
     _mode = _session.status == ActiveWorkoutStatus.running
         ? WorkoutExecutionMode.active
         : WorkoutExecutionMode.paused;
+    _reps = _initialRepsFor(_exercise);
     _seriesStartedAt = DateTime.now().toUtc();
     _ticker = Timer.periodic(const Duration(seconds: 1), (_) {
       if (mounted) setState(() {});
@@ -808,7 +809,7 @@ final class _WorkoutExecutionScreenState extends State<WorkoutExecutionScreen> {
 
   Future<void> _moveTo(ExecutionPosition position) async {
     _scoreController.clear();
-    _reps = 0;
+    _reps = _initialRepsFor(_plan.exerciseAt(position));
     _activeRestStateId = null;
     _scoreStopwatch = null;
     _manualScoreTimeMs = null;
@@ -889,6 +890,11 @@ final class _WorkoutExecutionScreenState extends State<WorkoutExecutionScreen> {
       ),
     );
   }
+
+  int _initialRepsFor(ExecutionExercise exercise) {
+    if (!exercise.repsEnabled) return 0;
+    return exercise.targetReps ?? 0;
+  }
 }
 
 final class SetMeasureInput extends StatelessWidget {