Do not deploy the empty kanshi config

defaults/xdg/kanshi/config is a 0-byte placeholder, unused on Arch as
well, where kanshi reads ~/.config/kanshi/config instead. Deploying it
made kanshi fail to parse and restart in a loop. The unit now starts only
when a user config exists.

modules/desktop.nix

@@ -65,7 +65,10 @@ in
     "xdg/quickshell/moonarch".source = "${xdg}/quickshell/moonarch";
     "xdg/foot/foot.ini".source = "${xdg}/foot/foot.ini";
     "xdg/fastfetch/config.jsonc".source = "${xdg}/fastfetch/config.jsonc";
-    "xdg/kanshi/config".source = "${xdg}/kanshi/config";
+    # defaults/xdg/kanshi/config is an empty placeholder — on Arch too. kanshi
+    # rejects it ("failed to parse config file") and restarts in a loop, so it
+    # is not deployed. Monitor profiles are machine specific and belong in
+    # ~/.config/kanshi/config.
     "xdg/Kvantum/kvantum.kvconfig".source = "${xdg}/Kvantum/kvantum.kvconfig";
     "xdg/qt6ct/qt6ct.conf".source = "${xdg}/qt6ct/qt6ct.conf";
     "xdg/gtk-3.0/settings.ini".source = "${xdg}/gtk-3.0/settings.ini";

modules/services.nix

@@ -34,6 +34,9 @@
     partOf = [ "graphical-session.target" ];
     after = [ "graphical-session.target" ];
     wantedBy = [ "graphical-session.target" ];
+    # Without a config kanshi exits with a parse error and, with
+    # Restart=on-failure, spins. Monitor profiles are per machine.
+    unitConfig.ConditionPathExists = "%h/.config/kanshi/config";
     serviceConfig = {
       Type = "simple";
       ExecStart = lib.getExe pkgs.kanshi;