feat: expose player over MPRIS for system media controls

Register a session D-Bus MPRIS service (org.mpris.MediaPlayer2.mtsmusic)
in both the TUI and Qt apps, so the player shows up as a media source:
play/pause, next/previous, seek, metadata (title/artist/album/length/art)
and volume can be controlled from the GNOME media-controls extension,
playerctl and other MPRIS clients. Integration is optional and degrades
to a no-op without dbus-next or a session bus.

requirements.txt

@@ -2,3 +2,4 @@ playwright>=1.40
 requests>=2.31
 textual>=0.80
 pillow>=10
+dbus-next>=0.2.3