feat: add background audio Pomodoro prototype

[MINBBBIGcode]

Refs #1.

Summary:

• Add a single-file Pomodoro timer prototype focused on reliable Chrome background audio behavior.
• Include audio unlock handling, notification fallback, and a small smoke test.
• Add testing notes for manual browser verification.

Validation:

• node smoke-test.js
• npx --yes playwright screenshot file:///mnt/c/Users/happy/Desktop/openclaw/sub%20cost/work/OSS-bounties/pomodoro-timer.html output/playwright/pomodoro-desktop.png
• Sensitive-data scan over changed files found no matches.