Fix SCSS syntax errors in promare-gradients variables

[Copilot]

Corrected SCSS syntax errors in _variables.scss that would prevent compilation:

• Fixed math.div() function call with closing parenthesis split across lines
• Changed square-root() to math.sqrt() (correct SCSS math module function)

Before:

$phi: math.div(square-root(5) + 1, 2
);

After:

$phi: math.div(math.sqrt(5) + 1, 2);

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