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/Core/Upload/UploadAPI.swift

@@ -6,7 +6,7 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 /// OSS 配置服务协议，抽象 STS token 获取能力，便于上传服务测试替换。
 @MainActor
@@ -16,10 +16,9 @@ protocol OSSConfigServing {
 }
 
 @MainActor
-@Observable
 /// 上传 API，封装文件上传前需要的服务端配置接口。
 final class UploadAPI {
-    @ObservationIgnored private let client: APIClient
+    private let client: APIClient
 
     /// 初始化上传 API，并注入共享网络客户端。
     init(client: APIClient) {