[FEAT] Add Export Functionality for Workout Sessions

[FalkenDev]

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

I want to keep a record of my workout sessions, but there is no way to export them from the app.

Describe the solution you'd like

I would like the ability to export all workout sessions to a JSON file.

This feature could be added to the "Training Journal" section with an "Export" button.

Describe alternatives you've considered

Manually recording my workout sessions in a spreadsheet, but this is time-consuming and inconvenient.

Why is this feature important to you?

This feature would allow me to back up my workout history and analyze it outside the app. 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.