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/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