fix: add checkbox and radio button field support in filler.py

[SouravVerma-art]

Fixes #229

What this PR does

• Added field type detection in src/filler.py for /Btn field types
• Checkbox and radio button fields are now correctly set to /Yes or /Off based on extracted values
• Existing text field behavior is completely unchanged
• Fixed src/main.py to read user input from input.txt instead of hardcoded text

Changes

• src/filler.py - Added /Btn field type handling
• src/main.py - Read user input dynamically from ./src/inputs/input.txt

Testing

• Tested locally with Docker
• Text fields continue to work correctly
• App runs end to end successfully

file_20260312_183347_filled.pdf

[SouravVerma-art]

Hi, I am interested in contributing to FireForm as a potential GSoC 2026 contributor.
I have started exploring the codebase and opened this PR as my first contribution.

I am also planning to work on the GSoC project idea related to this area.
Could you please let me know if there are other issues or improvements I should work on?