feat: add output directory support for saving plots and data

[eugenejyuan]

What's Changed

• Now saves 300 DPI PNG, PDF, and FSO data as TEXT file at output directory
• Fixed GUI results path consistency - now updates when you manually edit the path in the text field

Why

Previously, figures could only be saved via Matplotlib’s GUI “Save” button, resulting in blurry images, and the output_dir parameter was ignored.

Testing

✅ Tested on macOS - both GUI and CLI work great!