fix: Use example poster for reliable preview generation
- Replace random Jellyfin poster lookup with consistent example poster
- Remove complex get_random_unmodified_poster() function that was timing out
- Use demo data (DTS-HD MA, 4K HDR) for consistent preview experience
- Update frontend text to reflect example poster usage
- Improve preview reliability and speed by eliminating Jellyfin dependency
Fixes preview timeout issues when no unmodified posters are available.