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/App/State/AppSession.swift

@@ -6,7 +6,7 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 /// 登录阶段实体，表示根视图当前应该展示的认证状态。
 enum AuthPhase: Equatable {
@@ -16,11 +16,10 @@ enum AuthPhase: Equatable {
 }
 
 @MainActor
-@Observable
 /// 登录会话状态中心，保存 token 和当前认证阶段。
-final class AppSession {
-    private(set) var phase: AuthPhase = .loggedOut
-    private(set) var token: String?
+final class AppSession: ObservableObject {
+    @Published private(set) var phase: AuthPhase = .loggedOut
+    @Published private(set) var token: String?
 
     var isLoggedIn: Bool {
         phase == .loggedIn