feat: add background audio Pomodoro prototype

submissions/pomodoro-background-audio-wenyan/TESTING.md

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+# Pomodoro Background Audio Testing
+
+## Files
+
+- `index.html` contains the complete Pomodoro prototype with embedded CSS and JavaScript.
+- No external dependencies, build step, package manager, server, or database are required.
+
+## How to Run
+
+Open the file directly in recent Chrome desktop:
+
+```text
+submissions/pomodoro-background-audio-wenyan/index.html
+```
+
+Optional static server:
+
+```bash
+python3 -m http.server 8080
+```
+
+Then open:
+
+```text
+http://localhost:8080/submissions/pomodoro-background-audio-wenyan/index.html
+```
+
+## Self-Test
+
+Open the page with `?selftest=1` to run built-in deterministic tests for time formatting and input validation:
+
+```text
+index.html?selftest=1
+```
+
+Expected output includes:
+
+```text
+PASS format zero
+PASS format partial rounds up
+PASS format one minute
+PASS parse min
+PASS parse max
+PASS reject ""
+PASS reject "0"
+PASS reject "1000"
+PASS reject "1.5"
+PASS reject "abc"
+```
+
+## Manual Test Matrix
+
+### 1. Background Audio Test
+
+1. Set timer to `1` minute or temporarily edit the input using browser devtools for a shorter value during review.
+2. Press Start once. This unlocks Chrome Web Audio via a user gesture and schedules the completion chime on the audio clock.
+3. Switch to another Chrome tab or minimize Chrome.
+4. Wait until the timer should finish.
+5. Expected: the chime plays even while the page is backgrounded/minimized, and the display resets to the configured duration when visible again.
+
+### 2. Tab Inactive Accuracy
+
+1. Set timer to `1` minute.
+2. Press Start and immediately switch to another tab for about 30 seconds.
+3. Return to the timer tab.
+4. Expected: the display reflects wall-clock elapsed time, not throttled interval ticks.
+
+### 3. Pause/Resume
+
+1. Start a timer.
+2. Press Pause.
+3. Wait several seconds.
+4. Press Resume.
+5. Expected: countdown continues from the paused remaining time, not from the original duration.
+
+### 4. Input Validation
+
+Try these values and confirm a clear validation message appears:
+
+- Empty value
+- `0`
+- `-1`
+- `1.5`
+- `1000`
+- Non-numeric pasted text
+
+Expected: valid range is whole minutes from `1` to `999`.
+
+### 5. Audio Permission / Blocked Sound Scenario
+
+1. In Chrome site settings, block sound for the page origin if running through a local server, or mute the tab.
+2. Start and complete a timer.
+3. Expected: the timer still completes visually. If Web Audio cannot be started/resumed, the status area shows instructions to enable site sound and press Start again.
+
+## Implementation Notes
+
+- Countdown accuracy is based on a stored wall-clock deadline (`Date.now()`), so inactive-tab timer throttling does not accumulate drift.
+- Background audio reliability uses Web Audio scheduling. The alarm is scheduled at Start/Resume time after a user gesture unlocks the `AudioContext`, which is more reliable than waiting until a backgrounded JavaScript timer fires.
+- Wake Lock is requested when available, but it is optional. The timer remains correct without it.