add save-to-WAV button using OfflineAudioContext

[joeshub]

Renders the current sequence with OfflineAudioContext, encodes the
result as a 16-bit PCM WAV, and downloads it. WAV is simpler than
WebCodecs for uncompressed export and has no dependencies.

[Copilot]

Pull request overview

Adds offline audio rendering and WAV download support so users can export the current sequence as a 16‑bit PCM .wav file without extra dependencies.

Changes:

• Introduces renderSequenceToWav (OfflineAudioContext render) and WAV encoding + download helpers.
• Adds a “SAVE” button to the toolbar with loading/disabled state.
• Threads totalSteps/totalBeats through Board → Toolbar to compute export timing.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
src/utils/exportWav.js	Implements offline render, WAV encoding, and blob download helper.
src/components/Toolbar.jsx	Adds SAVE button and export handler using the new utility.
src/components/Toolbar.css	Styles SAVE button and disabled state.
src/Board.jsx	Passes timing parameters needed for export.

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[Copilot]

fetch(path) isn't checked for res.ok before decoding. If a sound file is missing or returns a non-audio response (404 HTML, etc.), decodeAudioData will fail with a less clear error. Consider throwing a descriptive error when !res.ok (including path and res.status).

Suggested change

	const res = await fetch(path)
	const res = await fetch(path)
	if (!res.ok) {
	throw new Error(`Failed to fetch audio file "${path}": ${res.status} ${res.statusText}`)
	}

[Copilot]

OfflineAudioContext is referenced as a global constructor here, but the codebase already uses vendor-prefixed AudioContext constructors for Safari compatibility. Consider selecting the constructor via window.OfflineAudioContext || window.webkitOfflineAudioContext (and failing with a clear error if unavailable) so export works in the same set of supported browsers.

[Copilot]

This uses the promise-returning form of decodeAudioData, but elsewhere in the codebase audio decoding is wrapped with the callback form (likely for Safari/WebKit compatibility). To avoid export breaking on browsers where decodeAudioData doesn't return a promise, use a small wrapper (or reuse the existing pattern) that supports the callback signature.

[Copilot]

DataView#setInt16 will coerce the float value here, which can truncate inconsistently across engines. Convert to a proper int (e.g., Math.round(...)) before writing so PCM values are stable and match expected WAV encoding.

Suggested change

	view.setInt16(offset, s < 0 ? s * 0x8000 : s * 0x7fff, true)
	const pcmSample = Math.round(s < 0 ? s * 0x8000 : s * 0x7fff)
	view.setInt16(offset, pcmSample, true)

[Copilot]

If renderSequenceToWav/fetch/decode fails, the error will bubble out of the click handler and show as an unhandled error in the console with no user feedback. Consider adding a catch branch to surface a message (and/or log) so failures are handled gracefully while still resetting isSaving.

Suggested change

	downloadBlob(blob, `${safeName}_${BPM}bpm.wav`);
	downloadBlob(blob, `${safeName}_${BPM}bpm.wav`);
	} catch (error) {
	console.error("Failed to save track.", error);
	alert("Unable to save the track right now. Please try again.");