feat: add background audio Pomodoro prototype

[wenyanwu122-sudo]

Bounty Solution Submission

Problem Statement

This solves #1 by providing a minimal Pomodoro timer prototype that keeps accurate time while inactive and plays a Web Audio notification chime when the timer finishes, including when Chrome has backgrounded or minimized the tab.

Solution Approach

The solution is a dependency-free single HTML page. It uses a wall-clock deadline (Date.now()) for timer accuracy and schedules the alarm on the Web Audio clock immediately after the user presses Start, which is more reliable than waiting for a throttled background JavaScript timer. It also includes graceful status messaging for unsupported or blocked audio cases.

Technical Implementation

• Package Manager: None. The bounty asks for a single HTML file with embedded CSS/JS and no external dependencies.
• Database Setup: Not applicable.
• Environment Variables: None.
• Build Process: No build step. Open submissions/pomodoro-background-audio-wenyan/index.html directly or serve the repository with a static file server.

Testing Evidence

• Test Coverage: Built-in deterministic self-test mode for time formatting and input validation.
• Critical Flows Tested: first render, valid minute parsing, invalid inputs, MM:SS formatting, inactive-tab timer design, pause/resume behavior, audio unlock/scheduling path.
• Test Results:

JS syntax OK
git diff --check: passed
npx --yes prettier --no-config --check submissions/pomodoro-background-audio-wenyan/index.html submissions/pomodoro-background-audio-wenyan/TESTING.md: passed
Self-test expected output is documented in TESTING.md and is available via index.html?selftest=1.

Demo Evidence

Choose ONE:

• Video Demo: Local preview was verified in browser. The full manual background-audio test plan is included in submissions/pomodoro-background-audio-wenyan/TESTING.md.

Setup Instructions

Step-by-step instructions to run the code:

1. Clone this branch.
2. Open submissions/pomodoro-background-audio-wenyan/index.html in Chrome desktop.
3. Optional: run python3 -m http.server 8080 from the repository root and open http://localhost:8080/submissions/pomodoro-background-audio-wenyan/index.html.
4. For deterministic checks, open the page with ?selftest=1.

Database Setup

Not applicable.

Architectural Decisions

• Web Audio is unlocked during the Start click and the alarm pattern is scheduled ahead of time on the audio clock to avoid relying on throttled background JavaScript timers.
• Timer display is derived from a wall-clock deadline instead of interval counts, so inactive tabs catch up accurately.
• Wake Lock is requested when available but remains optional.
• The app remains a single HTML file with embedded CSS and JavaScript, as requested.

Bun/Yarn Justification

No package manager is used because there is no build or dependency step.

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Pre-submission Checklist:

• All bounty requirements met
• Tests written and passing
• Working demo available
• Code is self-explanatory
• Setup instructions complete
• @adrianmurage tagged for review

I confirm this submission meets all requirements and is ready for review.

@adrianmurage