fix: handle division by zero in calculator

[quantcode-agent]

Summary

• Added a guard in src/calculator.ts divide() function that throws Error("Division by zero") when the divisor is zero
• Added test cases in test/calculator.test.ts for both normal division and the division-by-zero error case
• All 5 tests pass

Assumptions

• Throwing an error is the appropriate behavior for division by zero (rather than returning Infinity or NaN)
• The existing test file structure and testing framework (bun test) were preserved