feat(auth): allow anonymous download for global skills

Update handleDownload to bypass the authentication requirement when the
namespace is 'global'. This enables unauthenticated users to download
public global skills, consistent with the publicly accessible route.

web/src/pages/skill-detail.tsx

@@ -302,7 +302,7 @@ export function SkillDetailPage() {
   }
 
   const handleDownload = async () => {
-    if (!user) {
+    if (namespace!=='global' && !user) {
       requireLogin()
       return
     }