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/AppState.qml

@@ -77,6 +77,8 @@ Item {
         // The same rules as the terminal menu (toggle_component).
         if (id === "gaming" && !w.gaming) w.single = false;
         if (id === "single" && w.single) w.gaming = true;
+        if (id === "nvidia") w.bigpicture = w.nvidia;
+        if (id === "bigpicture" && w.bigpicture) w.nvidia = true;
         if (id === "machine") w.poweroff = w.machine;
         if (id === "poweroff" && w.poweroff) w.machine = true;
         if (id === "machine" && !w.machine) w.kpin = false;