refactor(ui): 错误弹窗挂载收成 presentingErrors modifier

五处逐字复制的 10 行 .alert 样板(窗口壳、三个 destination 包装、服务器
分区)各变一行;ErrorAlertPresenter 的挂载边缘收进 UIErrorAlert.swift,
照 WindowActivationPolicy 的 ViewModifier 先例。AppPresentation 文案
映射边缘不动,presenter 所有权与穿透面不变,零行为变化。

ShadowsocksX-NG2/App/MainWindowView.swift

@@ -99,16 +99,7 @@ struct MainWindowView: View {
     .onChange(of: route.destination) { _, destination in
       if destination != .rules { ruleDraft = nil }
     }
-    .alert(
-      "操作失败",
-      isPresented: Binding(
-        get: { shellActionErrors.isPresented },
-        set: { if !$0 { shellActionErrors.dismiss() } })
-    ) {
-      Button("好", role: .cancel) {}
-    } message: {
-      Text(shellActionErrors.message ?? "")
-    }
+    .presentingErrors(shellActionErrors)
     .alert(
       "诊断已导出",
       isPresented: Binding(

ShadowsocksX-NG2/App/ServersView.swift

@@ -61,16 +61,7 @@ struct ServersView: View {
       Button("创建") { commitNewGroup() }
       Button("取消", role: .cancel) { cancelNewGroup() }
     }
-    .alert(
-      "操作失败",
-      isPresented: Binding(
-        get: { errors.isPresented },
-        set: { if !$0 { errors.dismiss() } })
-    ) {
-      Button("好", role: .cancel) {}
-    } message: {
-      Text(errors.message ?? "")
-    }
+    .presentingErrors(errors)
     .confirmationDialog(
       deleteTitle,
       isPresented: Binding(

ShadowsocksX-NG2/App/UIErrorAlert.swift

@@ -1,4 +1,4 @@
-import Foundation
+import SwiftUI
 
 /// 主窗口共享的错误弹窗呈现状态（UI 持有的窗口状态，issue #41）：目录工作流
 /// module 以 typed error 上抛且不生成本地化文案，这里在呈现边缘统一转换。
@@ -20,3 +20,31 @@ final class ErrorAlertPresenter: ObservableObject {
     message = nil
   }
 }
+
+/// `ErrorAlertPresenter` 的唯一 alert 挂载边缘：标题固定「操作失败」，文案取
+/// presenter 当前消息。此前这段 10 行样板复制在窗口壳、destination 包装与
+/// 服务器分区共五处；挂载点各自独立呈现，互不冲突。
+private struct PresentingErrors: ViewModifier {
+  @ObservedObject var presenter: ErrorAlertPresenter
+
+  func body(content: Content) -> some View {
+    content
+      .alert(
+        "操作失败",
+        isPresented: Binding(
+          get: { presenter.isPresented },
+          set: { if !$0 { presenter.dismiss() } })
+      ) {
+        Button("好", role: .cancel) {}
+      } message: {
+        Text(presenter.message ?? "")
+      }
+  }
+}
+
+extension View {
+  /// 挂载共享错误弹窗：present 进 `presenter` 的失败由此统一弹出。
+  func presentingErrors(_ presenter: ErrorAlertPresenter) -> some View {
+    modifier(PresentingErrors(presenter: presenter))
+  }
+}

ShadowsocksX-NG2/App/WorkspaceDestinations.swift

@@ -31,16 +31,7 @@ struct WorkspaceHomeView: View {
       didOfferLegacyImport = true
       showLegacyImportSheet = true
     }
-    .alert(
-      "操作失败",
-      isPresented: Binding(
-        get: { errors.isPresented },
-        set: { if !$0 { errors.dismiss() } })
-    ) {
-      Button("好", role: .cancel) {}
-    } message: {
-      Text(errors.message ?? "")
-    }
+    .presentingErrors(errors)
   }
 }
 
@@ -55,16 +46,7 @@ struct WorkspaceSubscriptionsView: View {
       errors: errors,
       onNodesRemoved: onNodesRemoved
     )
-    .alert(
-      "操作失败",
-      isPresented: Binding(
-        get: { errors.isPresented },
-        set: { if !$0 { errors.dismiss() } })
-    ) {
-      Button("好", role: .cancel) {}
-    } message: {
-      Text(errors.message ?? "")
-    }
+    .presentingErrors(errors)
   }
 }
 
@@ -80,15 +62,6 @@ struct WorkspaceDiagnosticsView: View {
       errors: errors,
       clipboard: clipboard
     )
-    .alert(
-      "操作失败",
-      isPresented: Binding(
-        get: { errors.isPresented },
-        set: { if !$0 { errors.dismiss() } })
-    ) {
-      Button("好", role: .cancel) {}
-    } message: {
-      Text(errors.message ?? "")
-    }
+    .presentingErrors(errors)
   }
 }