Let \( P: y^{2}=4ax,\; a>0 \) be a parabola with focus \( S \). Let the tangents to the parabola \( P \) make an angle of \( \frac{\pi}{4} \) with the line \( y=3x+5 \) touch the parabola \( P \) at \( A \) and \( B \). Then the value of \( a \) for which \( A \), \( B \) and \( S \) are collinear is:
