$\sum_{k=1}^n \sin \left(a + (k-1)d \right) = \dfrac{\sin(dn/2) \sin(a+d(n-1)/2)}{\sin(d/2)}$
In your case, for the numerator $a = \dfrac{2}{2n}$ and $d = \dfrac{2}{2n}$. Hence, the numerator is $ \dfrac{\sin(1/2) \sin(2/2n+2/2n \times (n-1)/2)}{\sin(1/2n)} = \dfrac{\sin(1/2) \sin((n+1)/(2n))}{\sin(1/2n)}$
Similarly, the denominator gives $\dfrac{\sin^2(1/2)}{\sin(1/2n)}$ Hence, $\dfrac{\dfrac{\sin(1/2) \sin((n+1)/(2n))}{\sin(1/2n)}}{\dfrac{\sin^2(1/2)}{\sin(1/2n)}} = \dfrac{\sin((n+1)/(2n))}{\sin(1/2)} = \dfrac{\sin(1/2) \cos(1/2n) + \cos(1/2) \sin(1/2n)}{\sin(1/2)}$
The series expansion at $n= \infty$ gives us $1 + \dfrac{\cot(1/2)}{2n} - \dfrac1{8n^2} + \mathcal{O}(1/n^3)$ Hence, the desired limit is $\lim_{n \rightarrow \infty} \left(\dfrac{\sin((n+1)/(2n))}{\sin(1/2)} \right)^n = \lim_{n \rightarrow \infty} \left(1 + \dfrac{\cot(1/2)}{2n} - \dfrac1{8n^2} + \mathcal{O}(1/n^3) \right)^n\\ = \exp \left(\dfrac12 \cot\left( \dfrac12 \right) \right)$