Migrate from iOS 17 Observation to iOS 16-compatible Combine architecture.

Lower deployment target to iOS 16, replace @Observable with ObservableObject, add navigation and UI compatibility shims, and include login smoke UI tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

suixinkan/Features/Orders/API/OrdersAPI.swift

@@ -6,7 +6,7 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 @MainActor
 /// 订单服务协议，抽象订单列表、核销列表和核销操作以便测试替换。
@@ -59,10 +59,9 @@ protocol OrderServing {
 }
 
 @MainActor
-@Observable
 /// 订单 API，封装订单管理、核销订单和订单核销接口。
 final class OrdersAPI: OrderServing {
-    @ObservationIgnored private let client: APIClient
+    private let client: APIClient
 
     /// 初始化订单 API，并注入共享网络客户端。
     init(client: APIClient) {