从 iOS 17 Observation 迁移至 iOS 16 兼容的 Combine 架构

将最低部署版本降至 iOS 16，以 ObservableObject 替换 @Observable，新增导航与 UI 兼容层，并补充登录冒烟 UI 测试。

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

suixinkan/Core/Networking/APIClient.swift

@@ -6,7 +6,7 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 /// URLSession 抽象协议，用于让网络客户端支持测试替身。
 protocol URLSessionProtocol {
@@ -17,13 +17,12 @@ protocol URLSessionProtocol {
 extension URLSession: URLSessionProtocol {}
 
 @MainActor
-@Observable
 /// 统一网络请求客户端，负责构造请求、注入 token、校验响应和解析业务 Envelope。
 final class APIClient {
-    @ObservationIgnored private let session: URLSessionProtocol
-    @ObservationIgnored private let encoder: JSONEncoder
-    @ObservationIgnored private let decoder: JSONDecoder
-    @ObservationIgnored private var authTokenProvider: (() -> String?)?
+    private let session: URLSessionProtocol
+    private let encoder: JSONEncoder
+    private let decoder: JSONDecoder
+    private var authTokenProvider: (() -> String?)?
 
     private let environment: APIEnvironment
     private let appVersion: String