extract MprisService singleton, share player state between bar and lock screen

shell/services/MprisService.qml

@@ -0,0 +1,21 @@
+pragma Singleton
+
+import QtQuick
+import Quickshell.Services.Mpris
+
+QtObject {
+    id: root
+
+    readonly property var players: (Mpris.players.values ?? []).filter(p => p.trackTitle || p.playbackState === MprisPlaybackState.Playing || p.playbackState === MprisPlaybackState.Paused)
+    property int playerIdx: 0
+    readonly property MprisPlayer player: players[playerIdx] ?? players[0] ?? null
+    readonly property bool playing: player?.playbackState === MprisPlaybackState.Playing
+
+    // Reset index if current player disappears
+    onPlayersChanged: if (playerIdx >= players.length)
+        playerIdx = 0
+
+    function switchPlayer(idx) {
+        playerIdx = idx;
+    }
+}