已知函数\(f(x)=\left( 1+\dfrac{\cos x}{\sin x} \right){{\sin }^{2}}x+m\sin \left( x+\dfrac{{ }\!\!\pi\!\!{ }}{4} \right)\sin \left( x-\dfrac{{ }\!\!\pi\!\!{ }}{4} \right)\).
\((1)\)当\(m=0\)时,求\(f(x)\)在区间\(\left[ \dfrac{π}{8}, \dfrac{3π}{4}\right] \)上的取值范围;
\((2)\)当\(\tan α=2\)时,\(f(\alpha )=\dfrac{3}{5}\),求实数\(m\)的值.