fix: Replace dangerous eval with safe calculator implementation

[surajyog]

Summary

This PR addresses a critical security vulnerability by removing the dangerous eval() function and replacing it with a safe calculator implementation.

Changes

• ✅ Removed dangerousCalc function that used eval()
• ✅ Added safeCalculator function with proper input validation
• ✅ Implemented parentheses balancing checks
• ✅ Added comprehensive JSDoc documentation

Security Improvements

• Prevents arbitrary code execution
• Validates input to only allow mathematical expressions
• Providing clear error messages for invalid input
• Uses Function constructor with sanitization instead of eval

Testing

The new safeCalculator function:

• ✅ Handles basic arithmetic operations
• ✅ Supports parentheses
• ✅ Rejects malicious input
• ✅ Validates parentheses balancing