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/MainScanTabItem.swift

@@ -0,0 +1,19 @@
+//
+//  MainScanTabItem.swift
+//  suixinkan
+//
+
+import UIKit
+
+/// 主 TabBar 中间扫码核销入口。
+enum MainScanTabItem {
+
+    /// 构建扫码 TabBarItem，点击行为由 `MainTabBarController` 拦截。
+    static func makeTabBarItem() -> UITabBarItem {
+        let image = UIImage(named: "icon_scan")?.withRenderingMode(.alwaysOriginal)
+        let item = UITabBarItem(title: nil, image: image, selectedImage: image)
+        item.accessibilityLabel = "扫码核销"
+        item.accessibilityIdentifier = "main.scan"
+        return item
+    }
+}