[FEATURE] Currency Script | Refactor monolithic script into modules

[sue-lim]

The current script is monolithic and hard to maintain or test. It mixes responsibilities like API calls, currency conversion, and I/O handling.

Describe the solution you'd like
Refactor the script into the following modules:

api_handler.py: for API requests/responses
currencies.py: for currency list and search
converter.py: for conversion logic
main.py: to tie everything together

Describe alternatives
Keeping the current structure, however this is not scalable or testable.

Additional context
Modularizing will make it easier to maintain, add features, and write unit tests.

[sue-lim]

@DhanushNehru - please kindly assign this issue to me.
Many thanks