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/App/State/ToastCenter.swift

@@ -5,17 +5,16 @@
 //  Created by Codex on 2026/6/18.
 //
 
-import Observation
+import Combine
 import SwiftUI
 
 @MainActor
-@Observable
 /// 全局 Toast 状态中心，负责保存和清除当前提示文案。
-final class ToastCenter {
-    fileprivate var message: String?
-    @ObservationIgnored private let autoDismissNanoseconds: UInt64
-    @ObservationIgnored private var dismissTask: Task<Void, Never>?
-    @ObservationIgnored private var displayToken: UInt64 = 0
+final class ToastCenter: ObservableObject {
+    @Published fileprivate var message: String?
+    private let autoDismissNanoseconds: UInt64
+    @Published private var dismissTask: Task<Void, Never>?
+    @Published private var displayToken: UInt64 = 0
 
     /// 创建 Toast 状态中心，默认 2.2 秒后自动隐藏。
     init(autoDismissNanoseconds: UInt64 = 2_200_000_000) {
@@ -65,7 +64,7 @@ final class ToastCenter {
 
 /// 全局 Toast 叠加层修饰器，负责把 Toast 展示到页面顶部。
 private struct GlobalToastOverlay: ViewModifier {
-    @Environment(ToastCenter.self) private var toastCenter
+    @EnvironmentObject private var toastCenter: ToastCenter
 
     func body(content: Content) -> some View {
         content