notifs: use adaptive timer for timeStr instead of reactive binding

Replace the per-second reactive binding with an imperative timer
that adapts its interval based on notification age: 5s for <1min,
30s for <10min, 60s for <1h, 5min for <1d, 1h for older.

services/Notifs.qml

@@ -146,21 +146,37 @@ Singleton {
         property var locks: new Set()
 
         property date time: new Date()
-        readonly property string timeStr: {
+        property string timeStr: qsTr("now")
-            const diff = Time.date.getTime() - time.getTime();
+
+        function updateTimeStr(): void {
+            const diff = Date.now() - time.getTime();
             const m = Math.floor(diff / 60000);
 
-            if (m < 1)
+            if (m < 1) {
-                return qsTr("now");
+                timeStr = qsTr("now");
-
+                timeStrTimer.interval = 5000;
+            } else {
                 const h = Math.floor(m / 60);
                 const d = Math.floor(h / 24);
 
-            if (d > 0)
+                if (d > 0) {
-                return `${d}d`;
+                    timeStr = `${d}d`;
-            if (h > 0)
+                    timeStrTimer.interval = 3600000;
-                return `${h}h`;
+                } else if (h > 0) {
-            return `${m}m`;
+                    timeStr = `${h}h`;
+                    timeStrTimer.interval = 300000;
+                } else {
+                    timeStr = `${m}m`;
+                    timeStrTimer.interval = m < 10 ? 30000 : 60000;
+                }
+            }
+        }
+
+        readonly property Timer timeStrTimer: Timer {
+            running: !notif.closed
+            repeat: true
+            interval: 5000
+            onTriggered: notif.updateTimeStr()
         }
 
         property Notification notification