When the "Add file link" dialog is opened, the text field labeled "Link" should be focused automatically. This improves usability by allowing immediate typing without requiring a mouse click.

Steps to reproduce

1. Open JabRef
2. Create a new library: File → New library
3. Click the + button in the toolbar to add a new entry
4. In the entry editor, select the General tab
5. In the File section, click the + button
6. The "Add file link" dialog appears — the "Link" field is not focused

Expected behavior

The cursor should automatically be placed in the "Link" field when the dialog opens.

Good first issue

This is a simple UI improvement. Relevant code is in org.jabref.gui.linkedfile.LinkedFileEditDialog.