feat: 饭卡打印助手 Flutter 版（Phase 1/2：协议服务器 + macOS CUPS 真机打印）

- 协议与 C# 版 card-printer-agent 逐字段对齐（38200-38202 顺延、CORS *、串行任务队列、30 分钟保留）
- macOS CUPS 后端：lpoptions 队列发现 + lp 参数列表提交（LC_ALL=C 防本地化粘连坑）
- --mock 模拟模式 + bin/agent.dart headless 入口 + 测试卡生成工具
- 真机验收：HiTi CS-200e status 就绪 + 真实出卡；macOS 通用二进制构建通过
- App Sandbox 已关闭（服务需绑回环端口并调用 lp 子进程）；zaxprint URL scheme 已注册

test/widget_test.dart

@@ -0,0 +1,57 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:desktop_plugin/print/backend.dart';
+import 'package:desktop_plugin/print/mock_backend.dart';
+import 'package:desktop_plugin/server/print_queue.dart';
+import 'package:desktop_plugin/server/protocol.dart';
+
+void main() {
+  group('协议 DTO', () {
+    test('PrinterStatus.offline 序列化字段与前端约定对齐', () {
+      final json = PrinterStatus.offline('打印机未连接').toJson();
+      expect(json['online'], false);
+      expect(json['status'], 0x80);
+      expect(json['statusText'], '打印机未连接');
+      expect((json['ribbon'] as Map)['remain'], -1);
+      expect(json['cardLeft'], -1);
+      expect(json['connection'], 'USB');
+    });
+
+    test('PrintRequest.fromJson 解析 front/back/copies', () {
+      final req = PrintRequest.fromJson({
+        'copies': 2,
+        'front': {'color': 'data:image/png;base64,AAA', 'k': 'data:image/png;base64,BBB'},
+        'back': {'color': 'data:image/png;base64,CCC'},
+      });
+      expect(req.copies, 2);
+      expect(req.front!.color, 'data:image/png;base64,AAA');
+      expect(req.front!.k, 'data:image/png;base64,BBB');
+      expect(req.back!.color, 'data:image/png;base64,CCC');
+    });
+  });
+
+  group('打印队列', () {
+    test('缺少 front.color 提交即报错', () {
+      final queue = PrintQueue(MockBackend());
+      expect(
+        () => queue.submit(PrintRequest.fromJson({'copies': 1})),
+        throwsArgumentError,
+      );
+    });
+
+    test('mock 任务串行走完 printing -> done', () async {
+      final queue = PrintQueue(MockBackend());
+      final id = queue.submit(PrintRequest.fromJson({
+        'front': {'color': 'data:image/png;base64,AAA'},
+      }));
+      expect(id, 't1');
+      expect(queue.get(id)!.state, 'printing');
+      await Future<void>.delayed(const Duration(milliseconds: 2700));
+      expect(queue.get(id)!.state, 'done');
+    });
+  });
+
+  test('osBackendName 在 mac 上为 cups', () {
+    expect(osBackendName(), anyOf('cups', 'pavo'));
+  });
+}