feat(track): play-on-hover cover art, replacing double-click

Add a play overlay button shown on cover art hover that inserts the
track into the queue right after the current track and jumps to it,
so it takes priority over what's already queued. Replaces the
double-click-to-play interaction with a new playNow queue action.

src/styles/shell.css

@@ -951,3 +951,28 @@
 .sb-sec-link.active {
   color: var(--fg-1);
 }
+
+/* ============================================================
+   TRACK ROW — cover art play overlay
+   ============================================================ */
+.track-art-play {
+  position: absolute;
+  inset: 0;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  border: none;
+  border-radius: 4px;
+  background: rgba(0, 0, 0, 0.5);
+  color: var(--fg-1);
+  font-size: 16px;
+  cursor: pointer;
+  opacity: 0;
+  transition: opacity var(--dur-quick);
+}
+.track-art:hover .track-art-play {
+  opacity: 1;
+}
+.track-art-play:hover {
+  color: var(--lime);
+}