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/MessageCenter/Views/MessageCenterViews.swift

@@ -9,9 +9,9 @@ import SwiftUI
 
 /// 消息中心页面，展示消息列表、筛选、已读和删除入口。
 struct MessageCenterView: View {
-    @Environment(MessageCenterAPI.self) private var messageAPI
-    @Environment(ToastCenter.self) private var toastCenter
-    @State private var viewModel = MessageCenterViewModel()
+    @Environment(\.messageCenterAPI) private var messageAPI
+    @EnvironmentObject private var toastCenter: ToastCenter
+    @StateObject private var viewModel = MessageCenterViewModel()
     @State private var selectedMessage: MessageItem?
     @State private var processingMessageId: String?
 
@@ -80,7 +80,7 @@ struct MessageCenterView: View {
             ProgressView("加载中...")
                 .frame(maxWidth: .infinity, maxHeight: .infinity)
         } else if viewModel.loadFailed {
-            ContentUnavailableView {
+            AppContentUnavailableView {
                 Label("消息加载失败", systemImage: "exclamationmark.triangle")
             } description: {
                 Text(viewModel.loadFailureReason ?? "请稍后重试")
@@ -92,7 +92,7 @@ struct MessageCenterView: View {
             }
             .frame(maxWidth: .infinity, maxHeight: .infinity)
         } else if viewModel.messages.isEmpty {
-            ContentUnavailableView("暂无消息", systemImage: "bell", description: Text("暂无可查看的系统消息"))
+            AppContentUnavailableView("暂无消息", systemImage: "bell", description: Text("暂无可查看的系统消息"))
                 .frame(maxWidth: .infinity, maxHeight: .infinity)
         } else {
             ScrollView {