templates: improve accessibility, dark mode, and typography

Bump base font size from 0.8rem to 1rem (16px) to meet mobile
accessibility guidelines and avoid iOS auto-zoom on inputs.

Add CSS custom properties for all theme colors with a
prefers-color-scheme: dark media query so pages adapt to OS dark
mode. Component inline styles reference var(--hs-*) tokens so they
follow the scheme automatically.

Accessibility improvements:
- role="status" + aria-live="polite" on success boxes
- role="alert" + aria-live="assertive" on error boxes
- role="note" on warning boxes
- Visible focus rings via :focus-visible
- Link underlines (don't rely on color alone)
- SVG icons use currentColor for theme adaptation
- prefers-reduced-motion media query
- <main> landmark element wrapping page content
- Button styling with 44px min-height touch target
- List item spacing

Updates juanfont/headscale#3182