進捗が生で届く！SSEで作るリアルタイム通知

Create a single HTML page that demonstrates Server-Sent Events (SSE) for live progress updates. The page should simulate a long-running task (e.g., file processing) and show a progress bar that updates in real-time using SSE. Use a simple Node.js-like backend simulation via JavaScript's EventSource (or a mock that works without a server). Include a start button, a progress bar, a status message, and a completion alert. Use clean, modern UI with minimal CSS. Ensure the code is self-contained and works in a browser without external dependencies (except CDN for any library if needed).