fix: handle division by zero in calculator

[stooit]

Summary

• Fixes division-by-zero bug in src/calculator.ts by adding a guard that throws an Error("Division by zero") when the divisor is 0
• Adds test coverage for both normal division and the division-by-zero error case

Changes

• src/calculator.ts: Added zero-check in the divide function before performing division
• test/calculator.test.ts: Added tests for divide — verifying correct results and that dividing by zero throws

Assumptions

• Throwing an Error is the appropriate behaviour (rather than returning Infinity or NaN)
• The error message "Division by zero" is sufficient for consumers of this function

Testing

All 5 tests pass (bun test).