Add login flow, session routing, and MainTab shell aligned with reference UI.

Implement token-based root routing, Android-matched login screen, five-slot MainTabBar with scan action, and placeholder tab pages.

Co-authored-by: Cursor <cursoragent@cursor.com>

suixinkan/App/AppRouter.swift

@@ -0,0 +1,57 @@
+//
+//  AppRouter.swift
+//  suixinkan
+//
+
+import UIKit
+
+/// 应用根页面路由。
+enum AppRouter {
+
+    enum Root {
+        case login
+        case mainTab
+    }
+
+    /// 根据登录态创建根控制器：已登录 → 主页 Tab，未登录 → 登录页。
+    static func makeRootViewController() -> UIViewController {
+        makeViewController(for: AppStore.shared.isLoggedIn ? .mainTab : .login)
+    }
+
+    /// 切换窗口根控制器。
+    static func setRoot(_ root: Root, on window: UIWindow?, animated: Bool = true) {
+        setRoot(makeViewController(for: root), on: window, animated: animated)
+    }
+
+    /// 根据当前登录态刷新根控制器。
+    static func refreshRoot(on window: UIWindow?, animated: Bool = true) {
+        setRoot(AppStore.shared.isLoggedIn ? .mainTab : .login, on: window, animated: animated)
+    }
+
+    private static func makeViewController(for root: Root) -> UIViewController {
+        switch root {
+        case .login:
+            return UINavigationController(rootViewController: LoginViewController())
+        case .mainTab:
+            return MainTabBarController()
+        }
+    }
+
+    private static func setRoot(_ viewController: UIViewController, on window: UIWindow?, animated: Bool) {
+        guard let window else { return }
+
+        guard animated, let snapshot = window.snapshotView(afterScreenUpdates: true) else {
+            window.rootViewController = viewController
+            return
+        }
+
+        viewController.view.addSubview(snapshot)
+        window.rootViewController = viewController
+
+        UIView.animate(withDuration: 0.25, animations: {
+            snapshot.alpha = 0
+        }, completion: { _ in
+            snapshot.removeFromSuperview()
+        })
+    }
+}