fix(rundown): keep dragged entry visible across long drags

The rundown list is virtualised, so dragging an entry far enough for its
row to leave the render window unmounts the draggable. dnd-kit then loses
both the rect used for collision detection and the data attached to the
active element: the dragged element stopped being rendered and the drop
had no effect.

- render a drag overlay (portaled to the body) with a lightweight preview
  of the dragged entry, which keeps the drag visible and gives dnd-kit a
  stable rect for collision detection
- keep a copy of the active element data from drag start, and use it on
  drag end when dnd-kit falls back to empty data
- sortable entries stay in place as a dimmed placeholder while dragging,
  since the overlay now represents the moving element
- clear the drag state on drag cancel

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TEnUVARPnSqXijiz8KiLiE

apps/client/src/features/rundown/rundown-event/RundownEvent.tsx

@@ -253,7 +253,10 @@ export default function RundownEvent({
 
   const dragStyle = {
     zIndex: isDragging ? 2 : 'inherit',
-    transform: CSS.Translate.toString(transform),
+    // while dragging, the element is represented by the drag overlay
+    // we keep the original element in place as a placeholder
+    transform: isDragging ? undefined : CSS.Translate.toString(transform),
+    opacity: isDragging ? 0.4 : undefined,
     transition,
   };