Fix external StatusNotifierWatcher signal subscription on Arch CI

[Copilot]

The Arch Linux CI job was failing in systray smoke tests because external StatusNotifierWatcher registration/unregistration signals were not consistently observed. This made tests time out waiting for signal-driven item updates.

• Root cause

  • External watcher signal matching assumed sender='org.kde.StatusNotifierWatcher' only.
  • On some D-Bus setups, signal sender matching is enforced against the watcher’s unique owner (e.g. :1.42), so the match rule missed valid signals.

• Change

  • In SignalWatcher::spawn (WatcherMode::External), keep the existing match rule for the well-known sender.
  • Add a second match rule by resolving GetNameOwner(org.kde.StatusNotifierWatcher) and subscribing to that unique sender as well.
  • Behavior is unchanged for embedded watcher mode.

• Resulting behavior

  • External watcher registration/unregistration signals are consumed reliably across D-Bus implementations, preserving fast signal-path updates instead of falling back to reconcile timing.

add_match(&dbus, "type='signal',sender='org.kde.StatusNotifierWatcher',interface='org.kde.StatusNotifierWatcher'")?;
if let Ok(owner) = dbus.call::<_, _, String>("GetNameOwner", &(WATCHER_SERVICE,)) {
    let rule = format!("type='signal',sender='{owner}',interface='org.kde.StatusNotifierWatcher'");
    add_match(&dbus, &rule)?;
}