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
Extract shared HTML/CSS design into a common template and create
generalised auth success and web auth templates that work for both
node registration and SSH check authentication flows.
Updates #1850
Add design system assets for HTML templates:
- headscale.svg: Logo with optimized viewBox for proper alignment
- style.css: Material for MkDocs CSS variables and typography
- design.go: Design system constants for consistent styling
The logo viewBox is adjusted to 32.92 0 1247.08 640 to eliminate
whitespace from the original export and ensure left alignment with
text content.
