Slideshow HTML + JS v5

Fix v5:
- WebM export now preloads all images before recording.
- Canvas never draws empty frames.
- Export loop uses cached decoded images to avoid flashing.

Use:
1. Upload this folder to your website.
2. Put your pictures in assets/images/.
3. Edit manifest.json with the picture paths.
4. Open index.html from the website URL, not by double-clicking file://.

No PHP. No database. Static hosting only.
