Add trivia slideshow mode selection and optimize images

- Add mode selection screen: choose between game mode or slideshow mode
- Implement slideshow mode: clean photo gallery without trivia questions
- Add lightbox feature: click images to view full-screen
- Add keyboard navigation: arrow keys to navigate, Esc to close lightbox
- Optimize images: convert JPG to web-optimized PNG (256 colors)
- Reduce image sizes by 80% for faster page loading
- Update all image references to use optimized PNG files

Author: burinc