feat: drag-and-drop reordering for playlist tracks

Drag handle appears on hover, moves items in DOM, syncs order to
songs array, and saves to Navidrome via createPlaylist 800ms after
last drop (debounced to avoid rapid API calls).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

styles.css

@@ -189,6 +189,13 @@ nav{flex:1;overflow-y:auto;padding:4px 10px}
 .song-dur{font-size:11px;color:var(--text3);font-family:var(--mono)}
 .song-actions{display:flex;gap:2px;opacity:0;transition:opacity .15s}
 .song-item:hover .song-actions{opacity:1}
+/* ── PLAYLIST DRAG & DROP ── */
+.song-list.pl-draggable .song-item{grid-template-columns:18px 32px 44px 1fr auto}
+.pl-drag-handle{display:flex;align-items:center;justify-content:center;color:var(--text3);opacity:0;cursor:grab;transition:opacity .15s;touch-action:none}
+.song-list.pl-draggable .song-item:hover .pl-drag-handle{opacity:1}
+.pl-drag-handle:active{cursor:grabbing}
+.song-item.dragging{opacity:0.35}
+.song-item.drag-over{box-shadow:0 -2px 0 var(--accent)}
 
 /* ── ALBUM DETAIL ── */
 .back-btn{background:none;border:none;color:var(--text2);cursor:pointer;display:flex;align-items:center;gap:6px;font-family:var(--font);font-size:12px;padding:6px 0;margin-bottom:16px;transition:color .15s}