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/SessionBootstrapper.swift

@@ -6,14 +6,13 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 @MainActor
-@Observable
 /// 登录态启动恢复器，负责冷启动时从缓存恢复并校验 token。
 final class SessionBootstrapper {
-    @ObservationIgnored private let tokenStore: SessionTokenStore
-    @ObservationIgnored private let snapshotStore: AccountSnapshotStore
+    private let tokenStore: SessionTokenStore
+    private let snapshotStore: AccountSnapshotStore
     private var didAttemptRestore = false
 
     /// 初始化启动恢复器，并注入 token 与账号快照存储。