从 iOS 17 Observation 迁移至 iOS 16 兼容的 Combine 架构

将最低部署版本降至 iOS 16，以 ObservableObject 替换 @Observable，新增导航与 UI 兼容层，并补充登录冒烟 UI 测试。

Co-authored-by: Cursor <cursoragent@cursor.com>

suixinkan/Core/Design/GlobalLoadingCenter.swift

@@ -77,7 +77,7 @@ final class GlobalLoadingCenter {
 fileprivate final class GlobalLoadingState: ObservableObject {
     @Published fileprivate var isVisible = false
     @Published fileprivate var message = ""
-    fileprivate var activeCount = 0
+    @Published fileprivate var activeCount = 0
 }
 
 /// Lottie Loading 动画容器，负责播放主包中的 loading.json。