Add basic error handling and input validation layer

[BytecodeBrewer]

Goal

Create consistent input validation for amount and currency codes.

Why

CLI, API and GUI features will all need reliable validation. Centralizing this avoids duplicated checks across the project.

Scope

• Validate amount input
• Validate currency code format, e.g. EUR, USD, GBP
• Normalize currency codes to uppercase
• Return clear errors for invalid input
• Reuse validation in CLI and later GUI

Acceptance criteria

• Invalid amount input is handled
• Empty currency codes are rejected
• Currency codes are normalized consistently
• Validation logic is reusable outside CLI code
• Basic tests or example cases exist

Notes

This ticket supports both the API client and Tkinter GUI.

Note

Type: Feature / Refactor
Priority: Must
Area: Validation