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/ScenicSettlement/API/ScenicSettlementAPI.swift

@@ -6,7 +6,7 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 /// 景区结算服务协议，定义结算申请提交能力。
 @MainActor
@@ -16,10 +16,9 @@ protocol ScenicSettlementServing {
 }
 
 @MainActor
-@Observable
 /// 景区结算 API，封装结算申请提交网络请求。
 final class ScenicSettlementAPI {
-    @ObservationIgnored private let client: APIClient
+    private let client: APIClient
 
     /// 初始化景区结算 API，并注入共享网络客户端。
     init(client: APIClient) {