fix(security): replace new Function() with safe math expression parser (#753)

* fix(security): replace new Function() with safe math expression parser

Replace the dangerous new Function() constructor in evaluateMathExpression()
with a recursive descent parser that safely evaluates basic arithmetic
expressions.

The Function constructor is a security anti-pattern that could enable
arbitrary code execution if the validation regex is ever bypassed or
relaxed. The new parser:

- Only supports numbers, +, -, *, /, parentheses, and whitespace
- Has no eval-like functionality that could execute arbitrary code
- Maintains backward compatibility with existing expressions
- Handles operator precedence and parentheses correctly

Fixes #725

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Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

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Co-authored-by: Maze <167211895+mazeincoding@users.noreply.github.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

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