fix: extract stems from trimmed audio is using full song

src/util/audioUtil.ts

@@ -0,0 +1,52 @@
+function writeString(view: DataView, offset: number, str: string) {
+  for (let i = 0; i < str.length; i++)
+    view.setUint8(offset + i, str.charCodeAt(i));
+}
+
+export async function sliceAudioToWav(
+  arrayBuffer: ArrayBuffer,
+  startSeconds: number,
+  durationSeconds: number
+): Promise<ArrayBuffer> {
+  const audioCtx = new AudioContext();
+  const decoded = await audioCtx.decodeAudioData(arrayBuffer);
+  audioCtx.close();
+
+  const { sampleRate, numberOfChannels: numCh } = decoded;
+  const startSample = Math.floor(startSeconds * sampleRate);
+  const numSamples = Math.min(
+    Math.floor(durationSeconds * sampleRate),
+    decoded.length - startSample
+  );
+
+  const bytesPerSample = 2; // 16-bit PCM
+  const dataSize = numSamples * numCh * bytesPerSample;
+  const buf = new ArrayBuffer(44 + dataSize);
+  const view = new DataView(buf);
+
+  writeString(view, 0, 'RIFF');
+  view.setUint32(4, 36 + dataSize, true);
+  writeString(view, 8, 'WAVE');
+  writeString(view, 12, 'fmt ');
+  view.setUint32(16, 16, true);
+  view.setUint16(20, 1, true); // PCM
+  view.setUint16(22, numCh, true);
+  view.setUint32(24, sampleRate, true);
+  view.setUint32(28, sampleRate * numCh * bytesPerSample, true);
+  view.setUint16(32, numCh * bytesPerSample, true);
+  view.setUint16(34, 16, true);
+  writeString(view, 36, 'data');
+  view.setUint32(40, dataSize, true);
+
+  let offset = 44;
+  for (let i = 0; i < numSamples; i++) {
+    for (let ch = 0; ch < numCh; ch++) {
+      const s = decoded.getChannelData(ch)[startSample + i];
+      const c = Math.max(-1, Math.min(1, s));
+      view.setInt16(offset, c < 0 ? c * 32768 : c * 32767, true);
+      offset += 2;
+    }
+  }
+
+  return buf;
+}