Add drag-to-reorder within playlists

Cards in playlist view are draggable. Visual feedback: opacity on
source card, accent border on drop target. Reorder persists to store.

Files:
- frontend/app.js: dragSource state, drag/drop handlers on cards,
  container-level dragover guard in wireUI
- frontend/styles.css: .card.dragging, .card.drag-over styles

frontend/styles.css

@@ -751,6 +751,10 @@ input[type="range"]::-webkit-slider-thumb:hover { transform: scale(1.25); }
 .card:hover .card-menu { opacity: 1; }
 .card-menu:hover { color: #fff; border-color: var(--accent); background: var(--accent); }
 
+/* Drag-to-reorder */
+.card.dragging { opacity: 0.4; }
+.card.drag-over { border-color: var(--accent) !important; box-shadow: 0 0 0 2px var(--accent-glow); }
+
 /* Saved-offline badge on cards */
 .saved-badge {
   position: absolute; left: 5px; bottom: 5px;