fix: prevent overlay flicker during drag-and-drop

[Pixel998]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

This PR improves the editor’s drag-and-drop experience by eliminating overlay flicker while dragging within the editor and ensuring that dropping a file reliably inserts its content instead of triggering browser navigation.

What changes did you make? (Give an overview)

• Stabilized drag state by tracking drag depth, handling only file drags, and resetting on drop/dragend.
• Prevented navigation during drag/drop and simplified overlay visibility to remove flicker.

Related Issues

Fixes #128

Is there anything you'd like reviewers to focus on?

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[Copilot]

Pull Request Overview

This PR fixes overlay flicker during drag-and-drop operations in the editor by implementing a more robust drag state management system and preventing browser navigation during file drops.

• Enhanced the debounce utility function to include a cancel method for cleanup
• Implemented drag depth tracking to prevent flicker when dragging over nested elements
• Replaced manual DOM manipulation with proper React state updates for file content insertion

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/lib/utils.ts	Enhanced debounce function with cancel method and improved timeout typing
src/components/editor.tsx	Implemented drag depth tracking, simplified overlay logic, and improved drag-and-drop handling

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[nzakas]

LGTM. Thanks!