SLIDESHOW HTML + JS v4

This version fixes the export problem:
- The browser records from an internal canvas.
- It no longer tries stage.captureStream(), because a DIV cannot be recorded directly.

How to use:
1. Upload the whole folder to your website.
2. Open index.html through http/https, not by double-clicking file://.
3. Put your images in assets/images/.
4. Edit manifest.json.
5. Choose Landscape/Reel/Square/Portrait and click Export WebM.

No PHP. No database. No FFmpeg required for WebM export.
