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/Features/Auth/ViewModels/LoginViewModel.swift

@@ -6,7 +6,7 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 /// 登录输入焦点实体，标识当前需要聚焦的输入框。
 enum LoginField: Hashable {
@@ -68,17 +68,16 @@ enum LoginFlowError: LocalizedError {
 }
 
 @MainActor
-@Observable
 /// 登录页 ViewModel，负责表单状态、登录请求和多账号选择流程。
-final class LoginViewModel {
-    var username = "18651857230"
-    var password = "zhifly666"
-    var privacyChecked = false
-    var isLoading = false
-    var isSelectingAccount = false
-    var showsPassword = false
-    var showsAgreementSheet = false
-    var pendingAccountSelection: AccountSelectionPayload?
+final class LoginViewModel: ObservableObject {
+    @Published var username = "18651857230"
+    @Published var password = "zhifly666"
+    @Published var privacyChecked = false
+    @Published var isLoading = false
+    @Published var isSelectingAccount = false
+    @Published var showsPassword = false
+    @Published var showsAgreementSheet = false
+    @Published var pendingAccountSelection: AccountSelectionPayload?
 
     var canSubmit: Bool {
         isValidPhone && !trimmedPassword.isEmpty