feat(track): show now-playing bars overlay on cover art

Overlay the accent-coloured "hopping bars" PlayingIndicator on a
track's cover art wherever TrackRow appears when it's the active
player track, and reuse the same component/overlay for the current
entry's cover in the queue panel.

src/styles/shell.css

@@ -955,6 +955,14 @@
 /* ============================================================
    TRACK ROW — cover art play overlay
    ============================================================ */
+.track-art {
+  position: relative;
+  width: 36px;
+  height: 36px;
+  flex-shrink: 0;
+  border-radius: 4px;
+  overflow: hidden;
+}
 .track-art-play {
   position: absolute;
   inset: 0;
@@ -976,3 +984,27 @@
 .track-art-play:hover {
   color: var(--lime);
 }
+
+/* Now-playing overlay shown on a cover when its track is the active one
+   (track lists and the queue panel both use this). */
+.cover-playing {
+  position: absolute;
+  inset: 0;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: rgba(0, 0, 0, 0.45);
+}
+.track-art:hover .cover-playing {
+  opacity: 0;
+}
+
+/* Queue row cover-art wrapper, sized to match the 36px ArtTile */
+.qart {
+  position: relative;
+  width: 36px;
+  height: 36px;
+  flex-shrink: 0;
+  border-radius: 6px;
+  overflow: hidden;
+}