feat: DE selector with packagechooser, video wallpaper fix, slideshow redesign
- Replace dead init selector (packagechooser.conf, 303 lines) with
packagechooser_de.conf + packagechooser_dm.conf (netinstall-add method)
- Remove init SVGs (dinit, openrc, runit, s6)
- Clean netinstall.yaml — Desktop group removed, DEs added dynamically
- Update calamares-online/settings.conf with packagechooser@de + @dm instances
- Fix set-antergos-wallpaper.sh: Array() not [], no reloadConfig()
- Rewrite installer, launcher, and live-overlay docs