User background reflection fix applied.

Changes:
- User page now applies the background both through CSS variable and direct inline background-image.
- JavaScript fallback forces the selected background onto #userBg on page load.
- Slideshow mode also updates direct background-image.
- When a new background is uploaded in Admin Settings, the system automatically switches Background Mode to Fixed so the uploaded image shows immediately.

After uploading this ZIP to hosting:
1. Open Admin > Settings.
2. Upload your background under User Background.
3. Save Settings.
4. Open/refresh the user page in a private/incognito window or clear browser cache.
