osd/media playback: delay showing until album art is ready and avoid

re-showing when duplicate metadata comes from mpris
related #2787
port 1.5

quickshell/Services/TrackArtService.qml

@@ -191,6 +191,11 @@ Singleton {
         return (p.trackTitle || "") + "
" + (p.trackArtist || "") + "
" + (p.trackAlbum || "");
     }
 
+    function artReadyFor(player) {
+        const url = getArtworkUrl(player);
+        return url !== "" && url === _lastArtUrl && !loading && resolvedArtUrl !== "";
+    }
+
     function _updateArtUrl() {
         const key = _trackKey();
         // Skip once real art is committed for this track (dedup Chrome's multi-size