feat: add thumbnails and improve demo page styling

[mikelane]

Summary

• Split "dioxide" title into "di" (orange) + "oxide" (white) for visual distinction
• Add video thumbnails (160x90px) to all 5 demo cards
• Thumbnails extracted from each video at 5-second mark using ffmpeg
• Responsive design: thumbnails stack above content on mobile

Test plan

• Verified locally in browser (light and dark mode)
• All 5 thumbnails display correctly
• Cards remain clickable and link to demo pages
• Mobile responsive layout works

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.85%. Comparing base (b7a9190) to head (3b89e29).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #478   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files          21       21           
  Lines        1903     1903           
  Branches      372      372           
=======================================
  Hits         1748     1748           
  Misses         91       91           
  Partials       64       64           

Flag	Coverage Δ
python-3.13	91.85% <ø> (ø)

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