feat(V0.2): 拖动跟手浮层 —— 半透明副本随指针, 原位虚线占位

- useRowDrag/useHabitDrag 暴露 ghostRef, 拖动激活与移动时直接改
  transform (绕过 React 重渲染, 保证跟手流畅)
- 打卡浮层带颜色点 + 名称; 任务列表浮层带任务名 + 实时目标列名
- 原位卡片降为低透明度虚线占位, 落点指示不变

src/components/checkin/useHabitDrag.js

@@ -24,6 +24,13 @@ export default function useHabitDrag({ onDrop }) {
   const onDropRef = useRef(onDrop)
   onDropRef.current = onDrop
 
+  // 跟手浮层: 消费方渲染 <div ref={ghostRef}>, 拖动时 hook 直接改
+  // transform (绕过 React 重渲染, 保证跟手流畅)
+  const ghostRef = useRef(null)
+  const moveGhost = (x, y) => {
+    if (ghostRef.current) ghostRef.current.style.transform = `translate(${x}px, ${y}px) translate(-50%, -110%)`
+  }
+
   const cleanup = useCallback(() => {
     if (st.current.timer) { clearTimeout(st.current.timer); st.current.timer = null }
     st.current.active = false
@@ -58,6 +65,7 @@ export default function useHabitDrag({ onDrop }) {
       return
     }
     ev.preventDefault()
+    moveGhost(ev.clientX, ev.clientY)
     const hit = hitRow(ev.clientX, ev.clientY)
     if (hit) { s.over = hit; setOver(hit) }
     else if (s.over) { s.over = null; setOver(null) }
@@ -104,11 +112,13 @@ export default function useHabitDrag({ onDrop }) {
     if (instant) {
       st.current.active = true
       setDragItem(item)
+      moveGhost(st.current.startX, st.current.startY)
       if (navigator.vibrate) navigator.vibrate(15)
     } else {
       st.current.timer = setTimeout(() => {
         st.current.active = true
         setDragItem(item)
+        moveGhost(st.current.startX, st.current.startY)
         if (navigator.vibrate) navigator.vibrate(15)
       }, 400)
     }
@@ -137,5 +147,5 @@ export default function useHabitDrag({ onDrop }) {
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }), [])
 
-  return { dragItem, over, bind, bindHandle, cancel: cleanup, justEnded }
+  return { dragItem, over, bind, bindHandle, ghostRef, cancel: cleanup, justEnded }
 }