types: use Username() in User.Proto() when Name is empty

User.Proto() was returning u.Name directly, which is empty for OIDC
users who have their identifier in the Email field instead. This caused
"headscale nodes list" to show empty user names for OIDC-authenticated
nodes.

Only fall back to Username() when Name is empty, which provides a
display-friendly identifier (Email > ProviderIdentifier > ID). This
ensures OIDC users display their email while CLI users retain their
original Name.

Fixes #2972

hscontrol/types/users.go

@@ -174,9 +174,17 @@ func (u UserView) TailscaleUserProfile() tailcfg.UserProfile {
 }
 
 func (u *User) Proto() *v1.User {
+	// Use Name if set, otherwise fall back to Username() which provides
+	// a display-friendly identifier (Email > ProviderIdentifier > ID).
+	// This ensures OIDC users (who typically have empty Name) display
+	// their email, while CLI users retain their original Name.
+	name := u.Name
+	if name == "" {
+		name = u.Username()
+	}
 	return &v1.User{
 		Id:            uint64(u.ID),
-		Name:          u.Name,
+		Name:          name,
 		CreatedAt:     timestamppb.New(u.CreatedAt),
 		DisplayName:   u.DisplayName,
 		Email:         u.Email,