[FEAT] Add Export Functionality for Statistics (JSON and PDF)

[FalkenDev]

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

I want to analyze or share my progress, but there is no way to export my statistics from the app.

Describe the solution you'd like

I would like the ability to export my statistics in two formats:

1. JSON: A raw data file for programmatic analysis.
2. PDF: A designed report with:
   • Progress graphs and charts
   • Detailed session summaries
   • Key performance metrics (e.g., total weight lifted, average reps per session)

This feature could be added to the "Statistics" section with "Export JSON" and "Export PDF" buttons.

Describe alternatives you've considered

Manually creating reports in a spreadsheet, but this is time-consuming and lacks the visual appeal of a designed PDF.

Why is this feature important to you?

This feature would allow me to analyze my progress outside the app and share it with others. The PDF export would be especially useful for creating professional-looking reports.

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.