Keep notification QObjects out of the ListModels to stop shell crashes

Model rows stored the live Notification object in a ref role. When the
server destroyed the notification (sender close, DND untrack, dismiss)
the role became a dangling C++ pointer, and the next read segfaulted in
QQmlListModel::data — typically when replaying history over IPC. Track
live notifications in a JS map keyed by originalId instead, cleaned up
on close and untrack, where a stale reference degrades to a catchable
error instead of a crash.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

shell/plugins/notifications/NotificationLogic.js

@@ -73,8 +73,7 @@ function snapshotOf(notification, timestamp) {
     glyph: glyphFromHints(n.hints),
     urgency: n.urgency,
     expireTimeout: expireTimeout,
-    timestamp: timestamp === undefined ? Date.now() : timestamp,
-    ref: notification
+    timestamp: timestamp === undefined ? Date.now() : timestamp
   }
 }
 
@@ -91,8 +90,7 @@ function historyEntry(value, normalUrgency) {
     glyph: e.glyph || "",
     urgency: typeof e.urgency === "number" ? e.urgency : normalUrgency,
     expireTimeout: 0,
-    timestamp: e.timestamp || 0,
-    ref: null
+    timestamp: e.timestamp || 0
   }
 }