Fix runtime errors in main.py (Union import, Fill reference, target_f…

[lil-aditya]

Summary

This PR fixes several runtime errors in src/main.py that prevent the application from running correctly.

Issues Fixed

1. Missing Union Import

Union[str, os.PathLike] was used in a type hint but Union was not imported from typing, causing a NameError during module import.

2. Invalid Fill Reference

The function run_pdf_fill_process called Fill.fill_form(...) while the Fill import was commented out.
This resulted in a runtime NameError.

The implementation was updated to use the existing Controller class:

controller = Controller()
controller.fill_form(...)

Fixes #218