ghostty: prevent service restarts

Stopping or restarting the service will destroy open Ghostty sessions,
potentially disrupting work. This change ensures that we leave the
existing service untouched.

Fixes #8485

tests/modules/programs/ghostty/default.nix

@@ -1,5 +1,10 @@
+{ lib, pkgs, ... }:
+
 {
-  ghostty-example-settings = ./example-settings.nix;
   ghostty-empty-settings = ./empty-settings.nix;
+  ghostty-example-settings = ./example-settings.nix;
   ghostty-example-theme = ./example-theme.nix;
 }
+// lib.optionalAttrs pkgs.stdenv.hostPlatform.isLinux {
+  ghostty-systemd-service = ./systemd-service.nix;
+}

tests/modules/programs/ghostty/example-settings.nix

@@ -11,6 +11,9 @@
   };
 
   nmt.script = ''
+    servicePath=home-files/.config/systemd/user/app-com.mitchellh.ghostty.service
+    assertPathNotExists $servicePath
+
     assertFileContent \
       home-files/.config/ghostty/config \
       ${./example-config-expected}

tests/modules/programs/ghostty/systemd-service.nix

@@ -0,0 +1,28 @@
+{ config, ... }:
+{
+  programs.ghostty = {
+    enable = true;
+    package = config.lib.test.mkStubPackage { outPath = null; };
+    systemd.enable = true;
+    settings = {
+      theme = "catppuccin-mocha";
+      font-size = 10;
+    };
+  };
+
+  nmt.script = ''
+    servicePath=home-files/.config/systemd/user/app-com.mitchellh.ghostty.service
+    serviceOverridesPath=$servicePath.d/overrides.conf
+
+    assertFileExists $serviceOverridesPath
+    assertFileContent $serviceOverridesPath \
+      ${builtins.toFile "ghostty-service-overrides" ''
+        [Unit]
+        X-SwitchMethod=keep-old
+      ''}
+
+    assertFileContent \
+      home-files/.config/ghostty/config \
+      ${./example-config-expected}
+  '';
+}