fix(flutter): mini player art fills its 48dp box (BoxFit.cover)

Image with width+height but no fit paints the source at its intrinsic
resolution inside the box. The cover-cache thumbnails are smaller
than 48dp, so the art rendered as a tiny inset in the slate box
instead of filling it. Cover stretches/crops uniformly to fill.

flutter_client/lib/player/player_bar.dart

@@ -98,6 +98,11 @@ class _TrackInfo extends StatelessWidget {
                 : NetworkImage(media.artUri.toString()),
             width: 48,
             height: 48,
+            // Without a fit, Image paints the source at its intrinsic
+            // resolution inside the 48dp box — a thumbnail-sized cover
+            // would render as a tiny inset. Cover stretches/crops to
+            // fill the box uniformly.
+            fit: BoxFit.cover,
             errorBuilder: (_, __, ___) =>
                 Container(width: 48, height: 48, color: fs.slate),
           )