A function \( y = f(x) \) satisfies \( f(x)\sin 2x + \sin x - (1+\cos^2 x)f'(x) = 0 \) with condition \( f(0) = 0 \). Then, \( f\!\left(\frac{\pi}{2}\right) \) is equal to:
\( 1 \)
\( 0 \)
\( -1 \)
\( 2 \)