fix(mpris): remove required non-null artist/title from idle condition (#2993)

The current isIdle detection only works if the player has a title _and_ artist metadata,
which is not appropriate for all cases, e.g. MPV playing a video does not set artist.

In the current implementation, after a video stops playing in MPV, and the window closes
(but MPV and the MPRIS plugin server is still running as a background process), the playback controls remain
shown on the bar, and have non-functional controls.

It is sufficient to only check for the "Stopped" playing state. The MPRIS specification
states that Playback_Status==Stopped should be used to indicate that *NO* track is currently
playing. A compliant player should set that, and should also set the CanPlay/CanPause boolean
to false is not track is being played.

See: https://specifications.freedesktop.org/mpris/latest/Player_Interface.html#Enum:Playback_Status

quickshell/Services/MprisController.qml

@@ -168,7 +168,7 @@ Singleton {
     }
 
     function isIdle(player: MprisPlayer): bool {
-        return player && player.playbackState === MprisPlaybackState.Stopped && !player.trackTitle && !player.trackArtist;
+        return player && player.playbackState === MprisPlaybackState.Stopped;
     }
 
     // Known "<title> | <App>" suffixes stripped for matching only; display keeps the full title