MVP 1.3 Finalize currency input and precision rules #7

Requirements

• Use the precision provided by the exchange rate API, and handle conversion and display with money.js and accounting.js. No additional high-precision math libraries will be introduced.
• No maximum amount constraint. Accept any valid decimal number.
• Only allow standard decimal input. Scientific notation, letters, and special symbols are not permitted.

Implementation Tasks

• Immediate feedback for invalid input (display inline near the field)
• Unit tests using Vitest and Testing Library for valid and invalid inputs
• Update README under Validation & Precision section

Definition of Done (DoD)

• Valid input triggers conversion and UI updates correctly
• Invalid input is blocked and a clear error message is shown
• CI: npm run test:ci passes without failures