templates: add error box component and error page template

Add errorBox() and errorIcon() to the design system, mirroring the
existing successBox()/checkboxIcon() pattern with red error styling.
Extract error color constants from the inline values in statusMessage().

Add AuthError() template that renders a styled HTML error page using
the same HtmlStructure/mdTypesetBody/logo/footer as all other
browser-facing pages.

Updates juanfont/headscale#3182