UX overhaul: loading modal, instant domain switching, particle fixes, bounding boxes

- Add centered loading modal with spinner, progress bar, and site theme colors
- Remove loading indicator from domain switches (pre-load all bundles at boot)
- Use registry (index.json) as single source of truth for domain bounding boxes
- Recompute question-based bounding boxes via apply_flattened_coords.py
- Fix particle mouse dead zone caused by landing-domain-wrapper pointer-events
- Reduce particle repel radius to 1/4 previous size
- Enable auto-advance by default, revert non-auto modes after one answer
- Change skip button text to "Don't know (skip)"
- Add "Estimated Knowledge" label to screenshot colorbar
- Add extra newline before URL in share text
- Video markers: match article color/size, add click-to-play support

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

Author: jeremymanning