Your vector of frequencies is not correct, because fft returns the negative frequencies first, followed by the positive frequencies. I suggest looking at the documentation for fft.
Otherwise, trapz will integrate over whatever values you provide it. If you want it to integrate over a certain interval, only pass those elements as inputs. You can also look into integral().
Correction: fft returns the frequencies in the following order: zeroth frequency, positive frequencies up to the Nyquist freq, followed by the negative frequencies in ascending order.
