fix(media): sélecteur de média pour les exercices (ticket #15)

Corrige/ajoute le sélecteur de média sur exercise_library_screen.dart
suite aux retours de QA. flutter analyze propre (1 warning mineur
"paramètre inutilisé" dans un test, sans impact), 27/27 tests verts,
build APK debug validé.

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

test/presentation/exercise_library_screen_test.dart

@@ -65,6 +65,34 @@ void main() {
     expect(find.text('Répétitions'), findsOneWidget);
     expect(find.text('Score (kg)'), findsOneWidget);
   });
+
+  testWidgets(
+    'sélectionner une image déclenche l’import et confirme le fichier',
+    (tester) async {
+      final exerciseRepository = _FakeExerciseRepository();
+      final storage = _FakeMediaStorage();
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: ExerciseFormScreen(
+            exerciseUseCases: _exerciseUseCases(exerciseRepository),
+            mediaUseCases: _mediaUseCases(exerciseRepository, storage: storage),
+            mediaPicker: const _FakeMediaPicker(
+              imagePath: '/tmp/imports/photo.png',
+            ),
+          ),
+        ),
+      );
+
+      await tester.tap(find.text('Choisir une image'));
+      await tester.pump();
+
+      expect(storage.imports.single.sourcePath, '/tmp/imports/photo.png');
+      expect(storage.imports.single.kind, MediaKind.image);
+      expect(find.text('photo.png'), findsOneWidget);
+      expect(find.text('Image importée.'), findsOneWidget);
+    },
+  );
 }
 
 ExerciseUseCases _exerciseUseCases(_FakeExerciseRepository repository) {
@@ -76,17 +104,35 @@ ExerciseUseCases _exerciseUseCases(_FakeExerciseRepository repository) {
   );
 }
 
-MediaUseCases _mediaUseCases(_FakeExerciseRepository exerciseRepository) {
+MediaUseCases _mediaUseCases(
+  _FakeExerciseRepository exerciseRepository, {
+  _FakeMediaStorage? storage,
+}) {
   return MediaUseCases(
     mediaRepository: _FakeMediaAssetRepository(),
     exerciseRepository: exerciseRepository,
-    storage: _FakeMediaStorage(),
+    storage: storage ?? _FakeMediaStorage(),
     clock: _FakeClock(DateTime.utc(2026, 7, 17, 12)),
     ids: _FakeIds(),
     originDeviceId: 'device-1',
   );
 }
 
+final class _FakeMediaPicker implements MediaSourcePicker {
+  const _FakeMediaPicker({this.imagePath, this.videoPath});
+
+  final String? imagePath;
+  final String? videoPath;
+
+  @override
+  Future<String?> pickPath(MediaKind kind) async {
+    return switch (kind) {
+      MediaKind.image => imagePath,
+      MediaKind.video => videoPath,
+    };
+  }
+}
+
 EntityMetadata _metadata(String id) {
   return EntityMetadata(
     id: id,
@@ -167,6 +213,8 @@ final class _FakeMediaAssetRepository implements MediaAssetRepository {
 }
 
 final class _FakeMediaStorage implements LocalMediaStorage {
+  final imports = <_MediaImportCall>[];
+
   @override
   Future<void> deleteByLocalUri(String localUri) async {}
 
@@ -176,6 +224,13 @@ final class _FakeMediaStorage implements LocalMediaStorage {
     required MediaKind kind,
     required String stableFileName,
   }) async {
+    imports.add(
+      _MediaImportCall(
+        sourcePath: sourcePath,
+        kind: kind,
+        stableFileName: stableFileName,
+      ),
+    );
     return StoredMediaFile(
       localUri: 'file:///tmp/$stableFileName',
       mimeType: kind == MediaKind.image ? 'image/png' : 'video/mp4',
@@ -186,3 +241,15 @@ final class _FakeMediaStorage implements LocalMediaStorage {
   @override
   Future<Set<String>> listManagedLocalUris() async => const {};
 }
+
+final class _MediaImportCall {
+  const _MediaImportCall({
+    required this.sourcePath,
+    required this.kind,
+    required this.stableFileName,
+  });
+
+  final String sourcePath;
+  final MediaKind kind;
+  final String stableFileName;
+}