Dynamic slide numbering, fix notes editor, add screenshots, merge slides

- Auto-number visible slides from DOM (no more manual renumbering)
- Compute section boundaries from `data-section`/`data-budget` attributes
- Filter out `display:none` slides from navigation
- Fix notes editor: `e.preventDefault()` on N key so it doesn't type into textarea
- Remove `hidden` attribute from speaker-notes divs (CSS already hides them)
- Merge slides 5+6 into single "What AI needs" slide, drop misinformation line
- Split slide 9: separate Darkfish and no-live-preview points
- Add RBS HTML output screenshot replacing placeholder
- Hide Aliki features slide (considering merge into slide 11)
- Fix Aliki feature claims: responsive layout, improved search, ToC sidebar

Author: st0012