Redesign UI to Material Design 3 dark theme

- Full style.css rewrite with MD3 color token system (tonal surfaces,
  green/teal primary palette, shape scale, state layers)
- Roboto + Roboto Mono fonts loaded via Google Fonts
- Spool SVG hardcoded colors updated to match new green-tinted surfaces

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>