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/Tasks/API/TaskAPI.swift

@@ -6,7 +6,7 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 /// 任务服务协议，抽象任务列表、详情、发布和云盘选择接口以便测试替换。
 @MainActor
@@ -44,9 +44,8 @@ protocol TaskServing {
 
 /// 任务 API，封装任务管理和发布任务相关接口。
 @MainActor
-@Observable
 final class TaskAPI: TaskServing {
-    @ObservationIgnored private let client: APIClient
+    private let client: APIClient
 
     /// 初始化任务 API，并注入共享网络客户端。
     init(client: APIClient) {