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
3689f05407