fix: app crashed sometimes while converting qso in MP3

internal/audio/mp3.go

@@ -32,6 +32,20 @@ func writeMP3(path string, pcm []byte) error {
 	for i, v := range mono32 {
 		stereo[2*i], stereo[2*i+1] = v, v
 	}
+	// Shine's Write() reads a WHOLE frame (samplesPerPass × channels = 1152 × 2 =
+	// 2304 interleaved samples for MPEG-1) per pass via unsafe pointer arithmetic,
+	// regardless of how short the trailing chunk is. If the buffer isn't an exact
+	// multiple of a frame, the final pass reads past the slice and the process
+	// dies with an access violation (0xc0000005) inside windowFilterSubband.
+	// Pad with trailing silence to a whole number of frames so no partial pass
+	// exists. (~36 ms of silence at most — inaudible.)
+	const frameInterleaved = 1152 * 2 // samplesPerPass(MPEG-1) × 2 channels
+	if rem := len(stereo) % frameInterleaved; rem != 0 {
+		stereo = append(stereo, make([]int16, frameInterleaved-rem)...)
+	}
+	if len(stereo) == 0 {
+		return nil // nothing to encode (empty recording)
+	}
 	enc := mp3.NewEncoder(mp3Rate, 2)
 	return enc.Write(f, stereo)
 }