refactor: migrate state imports from contexts to stores directory

src/stores/user/index.ts

@@ -0,0 +1,11 @@
+/**
+ * @file User 公共导出
+ *
+ * 状态机重构后：
+ *   - 业务类（Dart 风格 context/reducer/side-effects）已合并到 user-machine.ts
+ *   - 类型由 user-types.ts 重新导出
+ *   - React 入口为 user-context.tsx（useMachine 包装）
+ */
+export * from "./user-context";
+export * from "./user-types";
+export { userMachine } from "./user-machine";