Add drag-and-drop file linking to comments tab in entry editor

[viru0909-dev]

Closes #10559

This PR adds support for dragging and dropping files (like images or PDFs) onto the 'Comments' tab in the entry editor. The feature automatically copies the dropped file to the library's main file directory and inserts a relative Markdown link at the cursor. This provides a much faster workflow for attaching and referencing files in comments.

Steps to test

1. Open any library and ensure a "main file directory" is set (e.g., via Library > Library properties > General file directory).
2. Select any entry and click on the "Comments" tab.
3. Drag an image file from your computer's desktop onto one of the comment text boxes.
4. Observe: A Markdown link (e.g., ![](image.png)) should immediately appear in the text box.
5. Verify: Check your library's file directory. The image you just dropped should now be copied there.

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• I added JUnit tests for changes (if applicable)
• I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[github-actions]

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[jabref-machine]

You committed your code on the main brach of your fork. This is a bad practice. The right way is to branch out from main, work on your patch/feature in that new branch, and then get that branch merged via the pull request (see GitHub flow).

For this pull request, this is OK. For subsequent pull requests, please start with a different branch with a proper branch name. See CONTRIBUTING.md for more details.

[jabref-machine]

Your code currently does not meet JabRef's code guidelines. IntelliJ auto format covers some cases. There seem to be issues with your code style and autoformat configuration. Please reformat your code (Ctrl+Alt+L) and commit, then push.

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