You are a senior front-end engineer.   Build a single-page Unit Converter that:  1. Lets the user pick a measurement category (Length, Weight, Temp, Volume) from a `<select>`. 2. Dynamically fills two unit `<select>` boxes for “From” and “To”. 3. Converts on input in real time—no submit button. 4. After each conversion, calls OpenAI /v1/chat/completions (gpt-4o) with:    “Explain one practical scenario where converting {{FROM}} → {{TO}} is useful, max 25 words.” 5. Shows the fun fact under the result. 6. Store `OPENAI_API_KEY` in `const OPENAI_API_KEY` at top. 7. CSS: neat flex layout, dark/light toggle via CSS variables. Return full HTML with inline `<style>` and `<script>`—nothing external.
Craft a fitness-inspired BMI Calculator featuring a vertical color bar that shifts from cool teal to warm coral according to the result. Incorporate large circular input dials, soft shadow layering, and a celebratory confetti burst when the ideal range is reached.