Implement first reusable FX API client

[BytecodeBrewer]

Goal

Create the first reusable API client for fetching exchange-rate data from the configured FX REST API.

Why

The project should move from experiment-level API usage toward a clean reusable component. This makes later CLI, GUI and visualization features easier to build on top.

Scope

• Create a small API client module, e.g. src/fx_converter_lab/clients/exchange_rate_client.py
• Fetch latest exchange rates for a selected base currency
• Return normalized Python data structures instead of raw response objects
• Handle basic request errors, invalid responses and unsupported currencies
• Keep API access separate from conversion logic

Acceptance criteria

• API client can fetch latest exchange rates for at least one base currency
• API response is converted into a clean internal structure
• Request errors produce understandable error messages
• Conversion logic does not directly call requests
• Tests exist
• Documentation exitsts

Note

Type: Feature
Priority: Must
Area: API / Backend Foundation