Only disable GPU in CI, update screenshots, fix modal screenshot delay

- Only disable GPU rendering when CI env var is set, not for all test
  runs. This ensures local screenshot generation uses GPU rendering
  for consistent results.
- Increase create-dandiset screenshot delay from 100ms to 500ms to
  wait for modal animation to complete.
- Regenerate all tutorial screenshots with GPU rendering enabled.

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

Author: rly