fix(admin): show anonymous user in Object Store Users UI (#8671)

The anonymous identity was explicitly filtered out of the user listing,
making it invisible in the admin console. Users could not view or edit
its permissions. Attempting to recreate it failed with "already exists".

Remove the anonymous skip in GetObjectStoreUsers so it appears like any
other identity. Add a guard in DeleteObjectStoreUser to prevent deletion
of the anonymous system identity, which would break unauthenticated S3
access.

Fixes #8466

Co-authored-by: Copilot <copilot@github.com>

weed/admin/dash/admin_server.go

@@ -882,11 +882,6 @@ func (s *AdminServer) GetObjectStoreUsers(ctx context.Context) ([]ObjectStoreUse
 
 	// Convert IAM identities to ObjectStoreUser format
 	for _, identity := range s3cfg.Identities {
-		// Skip anonymous identity
-		if identity.Name == "anonymous" {
-			continue
-		}
-
 		// Skip service accounts - they should not be parent users
 		if strings.HasPrefix(identity.Name, serviceAccountPrefix) {
 			continue