perf: optimize homepage images

- Convert team avatars and paper-card previews from raw <img> to next/image
  (responsive srcset, WebP/AVIF delivery, automatic lazy-loading below fold)
- Add scripts/optimize-images.mjs and downscale oversized source assets in
  place (team avatars to 320px, paper previews to 900px): 7.6MB -> 0.57MB

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ashtonanderson