Warn if file exists on export

[nogula]

Describe the bug A clear and concise description of what the bug is.

To Reproduce Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior A clear and concise description of what you expected to happen.

Screenshots If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• OS: [e.g. Windows 10]
• Python version [e.g., 3.11]
• matplotlib version [e.g., 3.9.1]
• PySide6 version [e.g., 6.7.2]
• FigureForge version [e.g. 0.1.0] These versions can be found in FigureForge by going to Help > Report Bug.

Additional context Add any other context about the problem here.

[nogula]

This dialog is raised if the user selects the file path via the browse button, but does not re-raise when the Export button is pressed. Therefore, the user can overwrite an existing file without necessarily knowing if they don't browse for the file (e.g., the previous export path is already filled in the text edit). This is the intended behavior.