refactor: simplify to 4 views + annotation toggle + per-image zoom

Redesign the HTML report modes based on user feedback:

Views: Both (side-by-side) | Base only | New only | Heatmap
- Removed overlay/swipe complexity in favor of clearer single-purpose views
- Each view shows exactly what it says — no ambiguity

Annotation toggle (key A):
- OFF: shows original images, thumbnails show new image
- ON: shows annotated versions (base_diff/diff), thumbnails show heatmap

Per-image zoom with drag-to-pan:
- Zoom buttons (+/-) scale each image independently inside its container
- At zoom > 100%, images overflow and users can drag to pan
- "Fit to width" resets to 100%
- Keyboard: +/- zoom, 0 reset

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pftg