Added comparison mode UI components to index.html:
- Diff toggle button (9th layer) for overlay visibility
- Comparison controls with sync scroll checkbox
- Side-by-side comparison container structure
These UI elements work with the existing comparison mode backend:
- /api/compare/document endpoint returns dual-document metadata
- /api/compare/page/{i} endpoint returns page data with diff
- /api/compare/page/{i}/svg/{side} endpoint renders SVG for each side
The diff overlay marks changes with color coding:
- Red: removed blocks (A only)
- Green: added blocks (B only)
- Yellow: changed blocks (both, but different)
Closes pdftract-1zg1h
