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/Common/AppButton.swift

@@ -0,0 +1,59 @@
+//
+//  AppButton.swift
+//  suixinkan
+//
+
+import SnapKit
+import UIKit
+
+/// 主按钮，对应 Android `AppButton`。
+final class AppButton: UIButton {
+
+    enum Style {
+        case primary
+        case secondary
+    }
+
+    private let style: Style
+
+    init(title: String, style: Style = .primary) {
+        self.style = style
+        super.init(frame: .zero)
+        setupUI(title: title)
+    }
+
+    @available(*, unavailable)
+    required init?(coder: NSCoder) {
+        fatalError("init(coder:) has not been implemented")
+    }
+
+    override var isEnabled: Bool {
+        didSet { updateAppearance() }
+    }
+
+    private func setupUI(title: String) {
+        titleLabel?.font = .systemFont(ofSize: 14, weight: .regular)
+        layer.cornerRadius = 10
+        clipsToBounds = true
+        setTitle(title, for: .normal)
+        updateAppearance()
+
+        snp.makeConstraints { make in
+            make.height.equalTo(46)
+        }
+    }
+
+    private func updateAppearance() {
+        switch style {
+        case .primary:
+            backgroundColor = isEnabled ? AppColor.primary : AppColor.buttonDisabled
+            setTitleColor(.white, for: .normal)
+            setTitleColor(.white, for: .disabled)
+        case .secondary:
+            backgroundColor = AppColor.secondaryButtonBackground
+            setTitleColor(AppColor.primary, for: .normal)
+            setTitleColor(AppColor.primary, for: .disabled)
+            alpha = isEnabled ? 1 : 0.6
+        }
+    }
+}