Merge a freshly-picked color with the previous color's alpha channel.
If picked is already in 8-char hex form (#rrggbbaa, length 9), it is
used as-is — its explicit alpha is preserved. Otherwise the alpha from
prev is adopted, which handles the common case where <input type="color">
strips the alpha channel and returns a bare 6-char hex.
Merge a freshly-picked color with the previous color's alpha channel. If
pickedis already in 8-char hex form (#rrggbbaa, length 9), it is used as-is — its explicit alpha is preserved. Otherwise the alpha fromprevis adopted, which handles the common case where<input type="color">strips the alpha channel and returns a bare 6-char hex.