fix: handle division by zero in calculator

[quantcode-agent]

Summary

• Fixed the divide function in src/calculator.ts to throw an Error("Division by zero") when the divisor is zero
• Added test cases in test/calculator.test.ts for normal division and division-by-zero error handling

Assumptions

• Throwing an error is the appropriate behavior for division by zero (rather than returning Infinity or NaN)
• The error message "Division by zero" is descriptive enough for consumers

All 5 tests pass.