Build a Stopwatch widget in React.

Requirements:

1. Display elapsed time in HH:MM:SS format.
2. Include buttons to start, stop, and reset the stopwatch.
3. Stopwatch should accurately track time even if paused and resumed.
4. Focus on functionality over styling.
5. The component should work out-of-the-box in a live React preview (e.g., CodeSandbox/Sandpack).