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/Scan/ScanPlaceholderViewController.swift

@@ -0,0 +1,50 @@
+//
+//  ScanPlaceholderViewController.swift
+//  suixinkan
+//
+
+import SnapKit
+import UIKit
+
+/// 扫码核销占位页，后续替换为真实扫码页。
+final class ScanPlaceholderViewController: BaseViewController {
+
+    private let titleLabel = UILabel()
+    private let closeButton = UIButton(type: .system)
+
+    override func setupNavigationBar() {
+        navigationController?.setNavigationBarHidden(true, animated: false)
+    }
+
+    override func setupUI() {
+        view.backgroundColor = .black
+
+        titleLabel.text = "扫码核销"
+        titleLabel.font = .systemFont(ofSize: 20, weight: .medium)
+        titleLabel.textColor = .white
+        titleLabel.textAlignment = .center
+
+        closeButton.setTitle("关闭", for: .normal)
+        closeButton.setTitleColor(.white, for: .normal)
+        closeButton.titleLabel?.font = .systemFont(ofSize: 16, weight: .medium)
+        closeButton.addTarget(self, action: #selector(closeTapped), for: .touchUpInside)
+
+        view.addSubview(titleLabel)
+        view.addSubview(closeButton)
+    }
+
+    override func setupConstraints() {
+        titleLabel.snp.makeConstraints { make in
+            make.center.equalToSuperview()
+        }
+
+        closeButton.snp.makeConstraints { make in
+            make.top.equalTo(view.safeAreaLayoutGuide).offset(12)
+            make.trailing.equalToSuperview().inset(16)
+        }
+    }
+
+    @objc private func closeTapped() {
+        dismiss(animated: true)
+    }
+}