feat: "Gaming on NVIDIA" replaces the SteamOS conversion on NVIDIA PCs

gamescope's gaming mode shows a corrupted picture on NVIDIA (NVIDIA bug 5240452), also nested in KDE and in a KWin-only
session. With an NVIDIA GPU the conversion (gaming, boot, single, glyphs) is hidden unless already on, and nvidia takes its
place: Steam installed when missing and started at login on the Plasma desktop (user unit), with the sub-option bigpicture
for Big Picture or the normal window. The kernel parameters and early NVIDIA modules (RTX 20 series or newer) are kept as
part of it. tests/nvidia-test.sh covers the component and the menu rules against a fake GPU.

lib/login-manager.sh

@@ -109,7 +109,6 @@ gaming_enable() {
     esac
     create_desktop_shortcut
     steam_enable
-    nvidia_enable || return 1
     os_name_enable || return 1
 }
 
@@ -125,7 +124,6 @@ gaming_disable() {
     steam_disable
     krevert gaming
     os_name_disable
-    nvidia_disable
     ok "The PC boots to the normal login screen again (from the next boot)."
 }