fix: handle division by zero in calculator

[quantcode-agent]

Summary

• Fixes division-by-zero bug in src/calculator.ts by throwing an Error("Division by zero is not allowed") when the divisor is 0
• Adds tests verifying both normal division and the division-by-zero error case
• All 5 tests pass

Assumptions

• Throwing an Error is the appropriate response for division by zero (rather than returning NaN/null)
• The error message "Division by zero is not allowed" is suitable for consumers of this function