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.

services/README.md

@@ -11,6 +11,7 @@ in the shell code.
 |---|---|---|
 | `steamify-boot-desktop.service` | `lib/boot-session.sh` (Boot into: desktop) | Sets the Plasma session before the login manager starts |
 | `sync-steamos-session.path`, `.service` | `lib/login-manager.sh` (SteamOS conversion, plasma-login-manager) | The sync bridge: copies Steam's session choice into `plasmalogin.conf` |
+| `steamify-steam-autostart.service` | `lib/nvidia.sh` (Gaming on NVIDIA) | Steam at login on the Plasma desktop, normal or in Big Picture (user unit) |
 | `steam-desktop-autostart.service` | `lib/steam-desktop.sh` (SteamOS conversion) | Steam in the background on the Plasma desktop (user unit) |
 | `ensure-kernel-headers.service` | `lib/steam-machine.sh` (Steam Machine support) | Installs missing kernel headers at boot, so DKMS builds for every kernel |
 | `steamify-update-check.service`, `.timer` | `lib/update-notifier.sh` (Update notifications) | The daily and login check for a new release (user units) |

services/steamify-steam-autostart.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Steam at login (Steamify, NVIDIA PCs)
+After=graphical-session.target
+PartOf=graphical-session.target
+
+[Service]
+Type=simple
+# Only in the Plasma desktop session.
+ExecCondition=/bin/sh -c '[ "$XDG_CURRENT_DESKTOP" = KDE ]'
+ExecStart=/usr/bin/steam @ARGS@
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=graphical-session.target