Background user-page fix applied and checked:
- index.php now applies the background inline with full shorthand fallback: background:#0f172a url(...) center/cover no-repeat !important
- JavaScript also reapplies background and CSS variable after page load.
- Slideshow mode reapplies full background shorthand on each slide.
- admin/background_check.php now uses the correct ../ path when previewing from the admin folder.
- CSS keeps object/background cover behavior and does not change the original UI layout.

Recommended server check:
1. Upload a JPG/PNG/WebP under Admin > Settings > User Background.
2. Save.
3. Click Background Check and confirm the preview image displays.
4. Click Preview User Page. Hard refresh browser (Ctrl+F5 / Cmd+Shift+R).
