[FEAT] Add Export Functionality for Exercises

[FalkenDev]

Is your feature request related to a problem? Please describe.

I want to back up or share my custom exercises, but there is currently no way to export them from the app.

Describe the solution you'd like

I would like the ability to export all exercises in my library to a JSON file.

This feature could be added to the "Exercise Library" section with an "Export" button.

Describe alternatives you've considered

Manually copying my exercises into a text file, but this is time-consuming and error-prone.

Why is this feature important to you?

This feature would allow me to back up my custom exercises and share them with others. It would also make it easier to migrate my data if I need to reinstall the app or move to a new device.

Additional context

None.

✅ Checklist

• I have searched the existing issues and have not found a similar feature request.
• I am willing to work on this feature myself. (Optional)
• I have read and agree to the project's Code of Conduct.