fix(ui): stop the album grid blanking its covers on refresh

Cover takes its height from react-measure, which reports nothing until it
re-measures. That was harmless while the cover class sat on the img itself,
which has intrinsic size; it now sits on the Artwork root, whose img fills it
absolutely and so lends no height. A refresh remounts the tiles, and for the
frame before measurement lands the box collapsed to zero and every cover
vanished.

Give the box its own aspect ratio as a floor. The measured height still wins
once it arrives.

ui/src/album/AlbumGridView.jsx

@@ -104,6 +104,9 @@ const useCoverStyles = makeStyles({
     display: 'inline-block',
     width: '100%',
     objectFit: 'contain',
+    // The image fills this box absolutely, so it lends no height: a remount that has not been
+    // re-measured yet would collapse the tile and blank the cover for a frame.
+    aspectRatio: '1',
     height: (props) => props.height,
     transition: 'opacity 0.3s ease-in-out',
   },