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/Profile/ViewModels/AccountSwitchViewModel.swift

@@ -6,17 +6,16 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 @MainActor
-@Observable
 /// 账号切换 ViewModel，管理可切换账号列表、选择状态和提交状态。
-final class AccountSwitchViewModel {
-    private(set) var accounts: [AccountSwitchAccount] = []
-    var selectedAccountId: String?
-    private(set) var loading = false
-    private(set) var switching = false
-    private var didLoad = false
+final class AccountSwitchViewModel: ObservableObject {
+    @Published private(set) var accounts: [AccountSwitchAccount] = []
+    @Published var selectedAccountId: String?
+    @Published private(set) var loading = false
+    @Published private(set) var switching = false
+    @Published private var didLoad = false
 
     /// 当前选中的账号。
     var selectedAccount: AccountSwitchAccount? {