fix(dnd): use pointerWithin collision detection for cross-container drag

Change from closestCenter to pointerWithin collision detection algorithm.
closestCenter calculates distance from item center to droppable centers,
which doesn't work well when the backlog is below the board - dragging
from backlog to columns often registered the wrong target.

pointerWithin uses cursor position to determine the drop target, which
is more intuitive for vertically stacked containers.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: msujaws

src/components/Board/Board.tsx

@@ -2,7 +2,7 @@ import { useMemo, useState, useEffect, useCallback } from 'react'
 import {
   DndContext,
   DragOverlay,
-  closestCenter,
+  pointerWithin,
   PointerSensor,
   useSensor,
   useSensors,
@@ -500,7 +500,7 @@ export function Board({
   return (
     <DndContext
       sensors={sensors}
-      collisionDetection={closestCenter}
+      collisionDetection={pointerWithin}
       onDragStart={handleDragStart}
       onDragEnd={handleDragEnd}
     >