fix: correct timeline layout bugs — desktop stacking, connector geometry, thumbnail...

fix: correct timeline layout bugs — desktop stacking, connector geometry, thumbnail ratio

- Remove flex from .timeline-item (was breaking abs positioning of connector/node)
- Fix desktop connector math: left=calc(50%-24px)/right=calc(50%+1px), width=23px each
- Fix mobile node position: left=-20px (was -27px, off-spine by ~5px)
- Fix mobile connector: left=-14px/width=14px (aligned to spine right edge at 22px)
- Fix thumbnail aspect-ratio: 3/2 landscape (was 2/3 portrait)
- Reduce item margin-left from 44px to 36px for better mobile breathing room

Co-Authored-By: Claude Haiku 4.5 <redacted>