Improve probo-agent tray icon and enrollment menu

Replace the placeholder orange-circle tray icon with the monochrome
Probo logo used in auditor-mode. On macOS, show the icon only (no
menu bar title); keep the title on Windows.

Restructure the context menu: status rows with colored indicators,
an Enroll via… submenu for region selection, and a clearer About
label. Surface enrollment errors in native dialogs instead of stderr.

Fix self-hosted enrollment on macOS by reading the hostname from
osascript's returned value; the default dialog output order broke
URL parsing and silently skipped opening the browser.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

pkg/deviceagent/tray/messagebox_windows.go

@@ -31,6 +31,7 @@ import (
 const (
 	mbOK              = 0x00000000
 	mbIconInformation = 0x00000040
+	mbIconError       = 0x00000010
 )
 
 var (