fix(studio): show where a dragged file will land instead of a drop-to-add box (#4142)

Dragging a file over Studio covered the screen with a "Drop to add at the
playhead" box, but a drop lands at the pointer time on the target row. The
box is gone; the timeline now draws a landing preview at the exact start and
track the drop will use (a new row below the last lane when that is the
target), taken from the same placement function the drop commits.

scripts/check-no-main-deletions.mjs

@@ -37,6 +37,10 @@ const BASE_FLAG = "--base";
  * its own. Remove an entry once its deletion has landed on the base.
  */
 export const ALLOWED_DELETIONS = new Map([
+  [
+    "packages/studio/src/components/StudioGlobalDragOverlay.tsx",
+    "the full-screen drop-to-add box is replaced by a landing preview on the timeline; no remaining references",
+  ],
   [
     "docs/snippets/catalog-overview-player.jsx",
     "#4051 removes the Catalog overview's only consumer of this snippet (replaced by the browse-grid mount); confirmed unreferenced repo-wide before deleting",