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/MainTabBadgeViewModel.swift

@@ -0,0 +1,21 @@
+//
+//  MainTabBadgeViewModel.swift
+//  suixinkan
+//
+
+import Foundation
+
+/// 主 Tab 角标 ViewModel，负责订单 Tab 待核销数量。
+final class MainTabBadgeViewModel {
+
+    var onChange: (() -> Void)?
+
+    private(set) var pendingWriteOffCount: Int? {
+        didSet { onChange?() }
+    }
+
+    /// 刷新待核销数量；接口未接入前保持为空。
+    func refreshPendingWriteOffCount() {
+        pendingWriteOffCount = nil
+    }
+}