fix: preserve queued settings and recover interrupted or corrupt state

mobile/lib/data/file_snapshot_store.dart

@@ -46,4 +46,18 @@ class FileSnapshotStore extends SnapshotStore {
     await temporary.rename(_file.path);
     return previous;
   }
+
+  @override
+  Future<AppSnapshot> startFresh() async {
+    await directory.create(recursive: true);
+    final stamp = DateTime.now().toUtc().microsecondsSinceEpoch;
+    for (final file in [_file, _backup]) {
+      if (await file.exists()) {
+        await file.rename('${file.path}.damaged-$stamp');
+      }
+    }
+    final fresh = AppSnapshot();
+    await save(fresh);
+    return fresh;
+  }
 }

mobile/lib/data/snapshot_store.dart

@@ -4,4 +4,5 @@ abstract class SnapshotStore {
   Future<AppSnapshot?> load();
   Future<void> save(AppSnapshot snapshot);
   Future<AppSnapshot> restoreBackup() => Future.error(StateError('没有可恢复的备份'));
+  Future<AppSnapshot> startFresh() => Future.error(StateError('无法重新建立保存文件'));
 }