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/main.go

@@ -12,12 +12,10 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
-	"syscall"
 	"time"
-	"unsafe"
 
-	"github.com/gin-gonic/gin"
 	"github.com/getlantern/systray"
+	"github.com/gin-gonic/gin"
 	"github.com/gofrs/flock"
 	"github.com/pressly/goose/v3"
 	"go.uber.org/zap"
@@ -447,49 +445,3 @@ func openBrowser(url string) error {
 
 	return cmd.Start()
 }
-
-func showError(title, message string) {
-	switch runtime.GOOS {
-	case "darwin":
-		script := fmt.Sprintf(`display dialog "%s" buttons {"OK"} default button "OK" with title "%s"`, message, title)
-		exec.Command("osascript", "-e", script).Run()
-	case "windows":
-		messageBox(title, message, MB_ICONERROR)
-	case "linux":
-		exec.Command("zenity", "--error", fmt.Sprintf("--title=%s", title), fmt.Sprintf("--text=%s", message)).Run()
-	}
-}
-
-func showAbout() {
-	message := "Nex Gateway\n\nAI Gateway - 统一的大模型 API 网关\n\nhttps://github.com/nex/gateway"
-	switch runtime.GOOS {
-	case "darwin":
-		script := fmt.Sprintf(`display dialog "%s" buttons {"OK"} default button "OK" with title "关于 Nex Gateway"`, message)
-		exec.Command("osascript", "-e", script).Run()
-	case "windows":
-		messageBox("关于 Nex Gateway", message, MB_ICONINFORMATION)
-	case "linux":
-		exec.Command("zenity", "--info", "--title=关于 Nex Gateway", fmt.Sprintf("--text=%s", message)).Run()
-	}
-}
-
-const (
-	MB_ICONERROR      = 0x10
-	MB_ICONINFORMATION = 0x40
-)
-
-var (
-	user32          = syscall.NewLazyDLL("user32.dll")
-	procMessageBoxW = user32.NewProc("MessageBoxW")
-)
-
-func messageBox(title, message string, flags uint) {
-	titlePtr, _ := syscall.UTF16PtrFromString(title)
-	messagePtr, _ := syscall.UTF16PtrFromString(message)
-	procMessageBoxW.Call(
-		0,
-		uintptr(unsafe.Pointer(messagePtr)),
-		uintptr(unsafe.Pointer(titlePtr)),
-		uintptr(flags),
-	)
-}