fix: handle division by zero in calculator

[quantcode-agent]

Summary

• Added a guard in src/calculator.ts to throw an error ("Cannot divide by zero") when the divisor is zero, instead of returning Infinity
• Added tests in test/calculator.test.ts to verify both normal division and the zero-divisor error case

Assumptions

• The appropriate behavior for division by zero is to throw an Error rather than returning a special value
• All existing tests continue to pass (5/5 passing)