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/Features/Main/Views/PlaceholderRootViews.swift

@@ -37,7 +37,7 @@ struct ProfileRootView: View {
 
 /// 通用 Tab 占位视图，用于未迁移模块的临时入口。
 private struct PlaceholderTabRootView: View {
-    @Environment(RouterPath.self) private var router
+    @EnvironmentObject private var router: RouterPath
 
     let title: String
     let systemImage: String