Act as a senior JavaScript developer with 5+ years of experience in building financial applications. Create a detailed step-by-step guide for building a dynamic currency converter using JavaScript, including fetching real-time exchange rates from an API. Cover the following: 1. Setting up the HTML structure with [INPUT_FIELDS] for amount and currency selection. 2. Integrating the [API_NAME] (e.g., ExchangeRate-API) for live data. 3. Writing JavaScript logic to handle [CURRENCY_PAIRS] (e.g., USD to EUR, GBP to JPY). 4. Implementing error handling for API failures or invalid inputs. 5. Adding a responsive design with [CSS_FRAMEWORK] (e.g., Bootstrap or Tailwind). Provide clean, commented code snippets and explain best practices for performance and security.