Consider migrating DropZone to VueUse's useDropZone

## Context

In PR #7734, a custom `DropZone` component was introduced at `src/renderer/extensions/linearMode/DropZone.vue` to handle drag-and-drop interactions.

## Suggestion

Use VueUse's [`useDropZone`](https://vueuse.org/core/useDropZone/#usedropzone) composable instead of the custom implementation for better maintainability and standardization.

## Blocker

The existing litegraph handlers are not compatible with `useDropZone`'s `checkValidity` function signature, making immediate migration not feasible.

## References

- PR: #7734
- Discussion: https://github.com/Comfy-Org/ComfyUI_frontend/pull/7734#discussion_r2673665454
- Requested by: @DrJKL

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-7971-Consider-migrating-DropZone-to-VueUse-s-useDropZone-2e66d73d36508179b743e348e1a96fe7) by [Unito](https://www.unito.io)