feat: add local backup export import core

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

test/presentation/home_screen_test.dart

@@ -243,6 +243,16 @@ final class _FakeBootstrap implements AppDependencies {
       ),
       exercisePerformanceReferenceUseCase = ExercisePerformanceReferenceUseCase(
         repository: _FakeExercisePerformanceReferenceRepository(),
+      ),
+      dataExportUseCase = DataExportUseCase(
+        repository: _FakeLocalDataBackupRepository(),
+        mediaStore: _FakeLocalBackupMediaStore(),
+        clock: _FakeClock(DateTime.utc(2026, 7, 17, 12)),
+      ),
+      dataImportUseCase = DataImportUseCase(
+        repository: _FakeLocalDataBackupRepository(),
+        mediaStore: _FakeLocalBackupMediaStore(),
+        clock: _FakeClock(DateTime.utc(2026, 7, 17, 12)),
       );
 
   @override
@@ -283,6 +293,12 @@ final class _FakeBootstrap implements AppDependencies {
 
   @override
   final ExercisePerformanceReferenceUseCase exercisePerformanceReferenceUseCase;
+
+  @override
+  final DataExportUseCase dataExportUseCase;
+
+  @override
+  final DataImportUseCase dataImportUseCase;
 }
 
 String _sessionSnapshot() {
@@ -682,6 +698,62 @@ final class _FakeMediaStorage implements LocalMediaStorage {
   Future<Set<String>> listManagedLocalUris() async => const {};
 }
 
+final class _FakeLocalDataBackupRepository
+    implements LocalDataBackupRepository {
+  @override
+  Future<LocalBackupImportResult> applyImportSnapshot({
+    required LocalDataExportSnapshot snapshot,
+    required LocalBackupImportMode mode,
+    required DateTime importedAt,
+  }) async {
+    return const LocalBackupImportResult(
+      insertedCount: 0,
+      updatedCount: 0,
+      ignoredOlderCount: 0,
+      deletedByReplaceCount: 0,
+      missingMediaCount: 0,
+    );
+  }
+
+  @override
+  Future<bool> hasAnyUserData() async => false;
+
+  @override
+  Future<bool> hasOpenActiveWorkoutSession() async => false;
+
+  @override
+  Future<LocalDataExportSnapshot> readExportSnapshot(
+    DateTime exportedAt,
+  ) async {
+    return LocalDataExportSnapshot(
+      exportedAt: exportedAt,
+      appSchemaVersion: 1,
+      originDeviceId: 'device-1',
+      mediaAssets: const [],
+      exercises: const [],
+      programs: const [],
+      workoutTemplates: const [],
+      workoutHistories: const [],
+    );
+  }
+}
+
+final class _FakeLocalBackupMediaStore implements LocalBackupMediaStore {
+  @override
+  Future<List<EmbeddedBackupMediaFile>> readEmbeddableFiles(
+    List<LocalBackupResource> mediaAssets,
+  ) async {
+    return const [];
+  }
+
+  @override
+  Future<Map<String, RestoredMediaFile>> restoreEmbeddedFiles(
+    List<EmbeddedBackupMediaFile> files,
+  ) async {
+    return const {};
+  }
+}
+
 final class _FakeProgramRepository implements ProgramRepository {
   @override
   Future<Program?> findById(String id) async => null;