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/UI/MainTab/AppTab.swift

@@ -0,0 +1,64 @@
+//
+//  AppTab.swift
+//  suixinkan
+//
+
+import UIKit
+
+/// 主 Tab 实体，描述底部导航中的一级入口。
+enum AppTab: String, CaseIterable, Hashable {
+    case home
+    case orders
+    case statistics
+    case profile
+
+    var title: String {
+        switch self {
+        case .home: "首页"
+        case .orders: "订单"
+        case .statistics: "数据"
+        case .profile: "我的"
+        }
+    }
+
+    var selectedImageName: String {
+        switch self {
+        case .home: "tab_home_selected"
+        case .orders: "tab_order_selected"
+        case .statistics: "tab_data_selected"
+        case .profile: "tab_profile_selected"
+        }
+    }
+
+    var unselectedImageName: String {
+        switch self {
+        case .home: "tab_home_unselected"
+        case .orders: "tab_order_unselected"
+        case .statistics: "tab_data_unselected"
+        case .profile: "tab_profile_unselected"
+        }
+    }
+
+    /// 构建系统 TabBarItem。
+    func makeTabBarItem() -> UITabBarItem {
+        let unselectedImage = UIImage(named: unselectedImageName)?.withRenderingMode(.alwaysOriginal)
+        let selectedImage = UIImage(named: selectedImageName)?.withRenderingMode(.alwaysOriginal)
+        let item = UITabBarItem(title: title, image: unselectedImage, selectedImage: selectedImage)
+        item.accessibilityIdentifier = "main.tab.\(rawValue)"
+        return item
+    }
+
+    /// 构建 Tab 根页面。
+    func makeRootViewController() -> UIViewController {
+        switch self {
+        case .home:
+            return HomeViewController()
+        case .orders:
+            return OrdersViewController()
+        case .statistics:
+            return StatisticsViewController()
+        case .profile:
+            return ProfileViewController()
+        }
+    }
+}