[FEATURE] Currency Script | Implement currency conversion logic in dedicated module

[sue-lim]

Conversion logic is currently embedded and lacks separation of concerns.

Describe the solution you'd like
Create a converter.py module that:

Accepts amount, source, and target currency
Calculates the converted amount using exchange rates

Describe alternatives
Keeping conversion logic inside main.py, but this violates separation of concerns.

Additional context
This will also help isolate the logic for unit testing.

[sue-lim]

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