Make the index page responsive, add dark mode

- Add viewport meta tag and text-size-adjust to fix Android font
  boosting that caused inconsistent text sizes on mobile
- Mobile layout (< 640px): CSS Grid header keeps title, date, and
  chevron in a clean row; tap-to-expand accordion reveals links as
  pill buttons; shortname hidden to save space
- Dark mode via prefers-color-scheme with GitHub-dark-inspired palette
- Non-breaking space between "Level" and number to prevent line breaks
- Activity dot tooltips, focus-visible outlines, iOS zoom prevention

Author: sideshowbarker