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.

ui/qml/Texts.qml

@@ -10,6 +10,12 @@ QtObject {
         boot: { label: "Boot into", hint: "Where the PC starts",
                 body: "Where the PC starts after a restart. Switching back and forth works the same either way.",
                 changes: ["Desktop: the Plasma session is set before login", "Gaming: the SteamOS default"] },
+        nvidia: { label: "Gaming on NVIDIA", hint: "Steam on the desktop, started at login",
+                  body: "gamescope's own gaming mode shows a corrupted picture on NVIDIA graphics cards (an open NVIDIA driver bug), so it isn't offered here. This puts Steam on the Plasma desktop instead and starts it when you log in.",
+                  changes: ["Steam is installed when it's missing", "Steam starts at login (a systemd user service)", "Off: the service is removed again; Steam is only removed if Steamify installed it"] },
+        bigpicture: { label: "Steam starts in Big Picture", hint: "The controller-friendly Steam, or the normal window",
+                      body: "Steam opens in Big Picture at login, which works well with a controller on the TV. Untick it for Steam's normal window.",
+                      changes: ["Steam is started with -gamepadui", "Off: Steam starts in its normal window"] },
         theme: { label: "SteamOS theme", hint: "Vapor look for the desktop, Add to Steam",
                  body: "The Vapor look of SteamOS for the desktop, with its panel, launcher icon and wallpaper.",
                  changes: ["cachyos-vapor global theme", "Add to Steam in right-click menus", "Nested Desktop, Steam keyboard window rule"] },