Fix slide layout: center content, bump sizes, split Darkfish/Aliki, fix diagrams

- Add `align-items: center` to `.slide` so content block is horizontally centered
- Increase font sizes ~35% and max-width caps for better fullscreen use
- Split Darkfish/Aliki into 2 separate slides with larger placeholders
- Fix Markdown coupling diagram to proper top-to-bottom flow
- Fix strikethrough diagram: all boxes on one line, no wrapping

Author: st0012