perf: consolidate notif timeStr timers into one global tick; fix osd open-on-start

modules/NotifService.qml

@@ -124,6 +124,15 @@ QtObject {
     }
 
     // Persistence
+    // Single global tick for all NotifItem.timeStr bindings — replaces per-item 5s timers
+    property real _now: Date.now()
+    property Timer _nowTimer: Timer {
+        running: root.count > 0
+        repeat: true
+        interval: 5000
+        onTriggered: root._now = Date.now()
+    }
+
     property Timer _saveTimer: Timer {
         interval: 1000
         onTriggered: {