Step 1: Use the Law of Sines.
Given that the angles of \( \triangle ABC \) are \( \frac{\pi}{4} \) and \( \frac{\pi}{3} \), we can use the Law of Sines to relate the sides of the triangle. The law states that:
\[
\frac{a}{\sin A} = \frac{b}{\sin B} = \frac{c}{\sin C},
\]
where \( A, B, C \) are the angles and \( a, b, c \) are the corresponding opposite sides.
Step 2: Find the ratio of the smallest and greatest sides.
By applying the Law of Sines, we find that the ratio of the smallest side to the greatest side is \( (\sqrt{3} - 1) : 1 \). Thus, the correct answer is option (B).