notifications: centralize icon resolution in NotifItem.resolvedIcon, fix lock pill grouping and missing fallbacks

shell/services/NotifItem.qml

@@ -1,4 +1,5 @@
 import QtQuick
+import Quickshell
 import Quickshell.Services.Notifications
 import "." as S
 
@@ -20,6 +21,17 @@ QtObject {
     property var actions: []
     property real time: Date.now()
 
+    // Resolved icon URL - checks image first, then resolves appIcon via XDG lookup
+    readonly property string resolvedIcon: {
+        if (image)
+            return image;
+        if (!appIcon)
+            return "";
+        if (appIcon.startsWith("/") || appIcon.startsWith("file://"))
+            return appIcon;
+        return Quickshell.iconPath(appIcon, "dialog-information");
+    }
+
     // Expire timer — owned by this item, not dynamically created
     readonly property Timer _expireTimer: Timer {
         running: false