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/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? {