fix: cmdPrev retry-skip + show SongCard actions on touch devices

Two corner-case fixes for the prev-history + play-next feature:

1. cmdPrev only tried queue.prev() once. instance.playNext's auto-
   advance retry-skip pushes failed songs into the same history stack,
   so a single prev frequently lands on an unplayable song — returning
   "Cannot play previous song" while leaving queue.currentIndex stuck
   mid-failure (causing next() to skip past the actually-playing song).
   Retry up to 4 times so prev finds a playable history entry, matching
   the retry budget already used by playNext for auto-advance.

2. SongCard.song-actions has opacity:0 by default and is revealed via
   parent :hover. Touch devices have no hover, so all three action
   buttons (Play / Play Next / Add) were invisible to phone/tablet
   users. Add @media (pointer: coarse) → opacity:1 to always show on
   touch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/components/SongCard.vue

@@ -156,6 +156,15 @@ function formatDuration(seconds: number): string {
   transition: opacity var(--transition-fast);
 }
 
+// Touch devices have no :hover, so the parent-hover-reveals-actions
+// pattern leaves all action buttons invisible. Always show on coarse-
+// pointer (touch) inputs — this is also where bigger tap targets matter.
+@media (pointer: coarse) {
+  .song-actions {
+    opacity: 1;
+  }
+}
+
 .action-btn {
   font-size: 18px;
   padding: 4px;