refactor: 桌面应用对话框代码拆分为平台专用文件

- 新增 dialog_windows.go、dialog_darwin.go、dialog_linux.go
- 使用 Go 构建标签实现条件编译
- 修复跨平台编译错误（syscall.NewLazyDLL 在 macOS/Linux 未定义）
- 实现 Linux 多工具降级策略（zenity → kdialog → notify-send → xmessage → stderr）
- 实现 macOS AppleScript 字符转义
- 更新 messagebox_test.go 构建标签
- 更新 desktop-app spec 新增 Linux 降级策略和 macOS 字符转义规范

backend/cmd/desktop/dialog_linux.go

@@ -0,0 +1,88 @@
+//go:build linux
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"sync"
+)
+
+type dialogToolType int
+
+const (
+	toolNone dialogToolType = iota
+	toolZenity
+	toolKdialog
+	toolNotifySend
+	toolXmessage
+)
+
+var (
+	dialogTool     dialogToolType
+	dialogToolOnce sync.Once
+)
+
+func init() {
+	dialogToolOnce.Do(detectDialogTool)
+}
+
+func detectDialogTool() {
+	tools := []struct {
+		name string
+		typ  dialogToolType
+	}{
+		{"zenity", toolZenity},
+		{"kdialog", toolKdialog},
+		{"notify-send", toolNotifySend},
+		{"xmessage", toolXmessage},
+	}
+
+	for _, tool := range tools {
+		if _, err := exec.LookPath(tool.name); err == nil {
+			dialogTool = tool.typ
+			return
+		}
+	}
+
+	dialogTool = toolNone
+}
+
+func showError(title, message string) {
+	switch dialogTool {
+	case toolZenity:
+		exec.Command("zenity", "--error",
+			fmt.Sprintf("--title=%s", title),
+			fmt.Sprintf("--text=%s", message)).Run()
+	case toolKdialog:
+		exec.Command("kdialog", "--error", message, "--title", title).Run()
+	case toolNotifySend:
+		exec.Command("notify-send", "-u", "critical", title, message).Run()
+	case toolXmessage:
+		exec.Command("xmessage", "-center",
+			fmt.Sprintf("%s: %s", title, message)).Run()
+	default:
+		fmt.Fprintf(os.Stderr, "错误: %s: %s\n", title, message)
+	}
+}
+
+func showAbout() {
+	message := "Nex Gateway\n\nAI Gateway - 统一的大模型 API 网关\n\nhttps://github.com/nex/gateway"
+
+	switch dialogTool {
+	case toolZenity:
+		exec.Command("zenity", "--info",
+			"--title=关于 Nex Gateway",
+			fmt.Sprintf("--text=%s", message)).Run()
+	case toolKdialog:
+		exec.Command("kdialog", "--msgbox", message, "--title", "关于 Nex Gateway").Run()
+	case toolNotifySend:
+		exec.Command("notify-send", "关于 Nex Gateway", message).Run()
+	case toolXmessage:
+		exec.Command("xmessage", "-center",
+			fmt.Sprintf("关于 Nex Gateway: %s", message)).Run()
+	default:
+		fmt.Fprintf(os.Stderr, "关于 Nex Gateway: %s\n", message)
+	}
+}