linux: switch systemd user service to type=notify-reload

This allows `systemctl` to send SIGUSR2 to Ghostty to trigger a reload,
which is more convenient than scripting `ps` and `kill` to find the
Ghostty main PID.

dist/linux/systemd.service.in

@@ -1,9 +1,12 @@
 [Unit]
 Description=@NAME@
 After=graphical-session.target
+After=dbus.socket
+Requires=dbus.socket
 
 [Service]
-Type=dbus
+Type=notify-reload
+ReloadSignal=SIGUSR2
 BusName=@APPID@
 ExecStart=@GHOSTTY@ --launched-from=systemd
 

src/apprt/gtk/App.zig

@@ -29,6 +29,7 @@ const apprt = @import("../../apprt.zig");
 const configpkg = @import("../../config.zig");
 const input = @import("../../input.zig");
 const internal_os = @import("../../os/main.zig");
+const systemd = @import("../../os/systemd.zig");
 const terminal = @import("../../terminal/main.zig");
 const Config = configpkg.Config;
 const CoreApp = @import("../../App.zig");
@@ -1035,6 +1036,12 @@ pub fn reloadConfig(
     target: apprt.action.Target,
     opts: apprt.action.ReloadConfig,
 ) !void {
+    // Tell systemd that reloading has started.
+    systemd.notify.reloading();
+
+    // When we exit this function tell systemd that reloading has finished.
+    defer systemd.notify.ready();
+
     if (opts.soft) {
         switch (target) {
             .app => try self.core_app.updateConfig(self, &self.config),
@@ -1367,6 +1374,9 @@ pub fn run(self: *App) !void {
         log.warn("error handling configuration changes err={}", .{err});
     };
 
+    // Tell systemd that we are ready.
+    systemd.notify.ready();
+
     while (self.running) {
         _ = glib.MainContext.iteration(self.ctx, 1);