Given positive real numbers $a$, $b$, and $c$ such that $a^2 + b^2 = c^2$ and $ab = c$. Determine the value of the expression:
\[
\left| \frac{(a+b+c)(a+b-c)(b+c-a)(c+a-b)}{c^2} \right|
\]

Let's think step by step and output the final answer within \boxed{}.
user