feat(client): App 内在线购买/续费授权（管理员支付宝下单+到账轮询）

- 新增套餐目录 license_plans.dart（标准/高级 × 月付/年付，与官网 checkout 一致）
- license 模型/仓库新增 PurchaseOrder/PurchaseStatusInfo 与下单、查单接口
- 系统设置授权面板：管理员显示「在线购买 / 续费」按钮，弹窗选套餐→
  外部浏览器打开支付宝收银台→3s 轮询到账→自动刷新授权并展示新到期日
- 用户手册（docs.md + user-manual.html）同步补充在线购买入口说明

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJ1g8XV1YhhmHRzhwWEW7o

client/lib/repositories/license_repository.dart

@@ -40,6 +40,34 @@ class LicenseRepository {
     }
   }
 
+  /// 在线购买/续费下单（仅管理员），返回 pay 收银台跳转信息。
+  Future<PurchaseOrder> createPurchase(String bizCode) async {
+    try {
+      final resp =
+          await _client.post('/license/purchase', data: {'biz_code': bizCode});
+      return PurchaseOrder.fromJson(resp.data['data'] as Map<String, dynamic>);
+    } on DioException catch (e) {
+      throw AppException(
+        e.response?.data?['error'] as String? ?? '下单失败，请稍后重试',
+        statusCode: e.response?.statusCode,
+      );
+    }
+  }
+
+  /// 查询购买单状态（到账轮询）。到账（paid）后返回续期后的授权到期时间。
+  Future<PurchaseStatusInfo> purchaseStatus(String outTradeNo) async {
+    try {
+      final resp = await _client.get('/license/purchase/$outTradeNo');
+      return PurchaseStatusInfo.fromJson(
+          resp.data['data'] as Map<String, dynamic>);
+    } on DioException catch (e) {
+      throw AppException(
+        e.response?.data?['error'] as String? ?? '查询订单状态失败',
+        statusCode: e.response?.statusCode,
+      );
+    }
+  }
+
   /// Deactivate (unbind) this device from its license.
   Future<void> deactivate() async {
     final deviceId = await DeviceId.get();