fix(notification): null _spring_timer_id after first remove in on_resolved

on_resolved removed the GLib source but didn't null the ID, causing
double-remove crash when destroy() later tried to clean up the same
stale source.

src/window/notification/notification.py

@@ -1005,6 +1005,7 @@ class NotificationRevealer(SlideRevealer):
         # Clean up any ongoing animations
         if self._spring_timer_id:
             GLib.source_remove(self._spring_timer_id)
+            self._spring_timer_id = None
 
         # Use different slide directions based on dismiss reason
         if reason == "expired":