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/AppUITestLaunchState.swift

@@ -0,0 +1,22 @@
+//
+//  AppUITestLaunchState.swift
+//  suixinkan
+//
+//  Created by Codex on 2026/6/26.
+//
+
+import Foundation
+
+/// UI 测试启动状态清理，仅在测试进程显式传入启动参数时执行。
+enum AppUITestLaunchState {
+    static let resetArgument = "-suixinkan-ui-tests-reset-state"
+
+    static func resetIfNeeded(arguments: [String] = ProcessInfo.processInfo.arguments) {
+        guard arguments.contains(resetArgument) else { return }
+
+        try? SessionTokenStore().clear()
+        AccountSnapshotStore().clear()
+        let preferences = AppPreferencesStore()
+        preferences.clear()
+    }
+}