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/Assets/API/AssetsAPI.swift

@@ -6,7 +6,7 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 /// 资产服务协议，抽象云盘、媒体库和相册接口以便 ViewModel 测试替换。
 @MainActor
@@ -83,9 +83,8 @@ protocol AssetsServing {
 
 /// 资产 API，负责封装相册云盘和素材管理相关接口。
 @MainActor
-@Observable
 final class AssetsAPI: AssetsServing {
-    @ObservationIgnored private let client: APIClient
+    private let client: APIClient
 
     /// 初始化资产 API，并注入共享网络客户端。
     init(client: APIClient) {