Regular octagon $ABCDEFGH$ has area $n$. Let $m$ be the area of quadrilateral $ACEG$. What is $\tfrac{m}{n}?$ $\textbf{(A) } \frac{\sqrt{2}}{4} \qquad$ $ \textbf{(B) } \frac{\sqrt{2}}{2} \qquad$ $ \textbf{(C) } \frac{3}{4} \qquad$ $ \textbf{(D) } \frac{3\sqrt{2}}{5} \qquad$ $ \textbf{(E) } \frac{2\sqrt{2}}{3}$