[FEATURE] Currency Script | Implement API handler for external exchange service

[sue-lim]

The API request and error handling are not abstracted, making it hard to reuse or test.

###Describe the solution you'd like
Create api_handler.py:

Fetch exchange rates
Handle errors (e.g., timeout, invalid key)
Return clean JSON data to the app

Describe alternatives
Inline API calls, which reduce flexibility and testability.

Additional context
This decouples API communication from app logic and allows mocking in tests.

[sue-lim]

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