Add httpUserError() alongside httpError() for browser-facing error
paths. It renders a styled HTML page using the AuthError template
instead of returning plain text. Technical error details stay in
server logs; the HTML page shows actionable messages derived from
the HTTP status code:
401/403 → "You are not authorized. Please contact your administrator."
410 → "Your session has expired. Please try again."
400-499 → "The request could not be processed. Please try again."
500+ → "Something went wrong. Please try again later."
Convert all httpError calls in oidc.go (OIDC callback, SSH check,
registration confirm) to httpUserError. Machine-facing endpoints
(noise, verify, key, health, debug) are unchanged.
Fixesjuanfont/headscale#3182