Use in-place source move for drag instead of placeholder + display:none

The clone-placeholder approach hid the source element with
display:none, which some browsers treat as drag cancellation. The drag
appeared to "not respond at all" to mouse press.

Switch to a simpler approach: keep the source element in the DOM and
move it directly during dragover. Apply a .dragGhost class after the
drag image is captured (via setTimeout 0) so the source becomes a
translucent dashed placeholder at the prospective drop position. drop
just compares the source's current DOM index to its original
data-index.

Also add cursor:grabbing on :active for visible press feedback.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

public/styles.css

@@ -440,16 +440,16 @@ body.siteBody.centerLayout {
 }
 .rowDur { color: var(--text-muted); font-size: 13px; }
 
-/* 드래그 시스템 공통: 원본은 잠시 숨기고, 같은 모양의 placeholder 가 들어갈 자리에서 반투명하게 보임 */
-.hiddenWhileDragging { display: none !important; }
-.dragPlaceholder {
+/* 드래그 시스템: 원본 요소가 그대로 새 위치로 이동하면서 반투명 ghost 로 보임 */
+.dragGhost {
   opacity: 0.45;
-  pointer-events: none;
   outline: 2px dashed var(--accent);
   outline-offset: -2px;
   background: rgba(47, 129, 247, 0.08);
 }
-.dragPlaceholder * { pointer-events: none !important; }
+.dragGhost * { pointer-events: none !important; }
+.trackRow:active { cursor: grabbing; }
+.imageCard:active { cursor: grabbing; }
 
 /* 사진 그리드 */
 .imageGrid {