Act as a senior JavaScript developer with 5+ years of experience in building health applications. Create a detailed step-by-step guide for building a BMI calculator using vanilla JavaScript. Include: 1) HTML structure with [INPUT_FIELDS] for height (cm/inches) and weight (kg/lbs), 2) JavaScript logic to [CALCULATE_BMI] using the formula BMI = weight(kg) / (height(m))^2 with unit conversion, and 3) [DISPLAY_RESULTS] with color-coded categories (Underweight, Normal, Overweight, Obese). Ensure the calculator handles edge cases like invalid inputs and provides a responsive UI. Add clear comments explaining each code block for educational purposes.