fix: resolve failing tests and implement missing utility functions

[stooit]

Summary

Takes bun test from 44 passing / 16 failing to 60 passing / 0 failing. tsc --noEmit passes. No test files were changed and no dependencies were added.

• calculator: divide now throws on division by zero instead of returning Infinity.
• string-utils: wordCount now splits on any run of whitespace. truncate is now implemented: it cuts at a word boundary, appends ..., and never goes over maxLength.
• task-manager: remove, update and sortBy are now implemented. sortBy sorts a copy, so the stored tasks keep their order.
• date-utils: formatRelative now uses Math.round for days, the same as it does for minutes and hours.
• validator: isEmail now accepts TLDs longer than 4 characters. isUrl now accepts URLs that include a port.

Assumptions / notes

• divide can now throw, which is a breaking change for any caller that expected Infinity. Nothing in this repo relies on the old behaviour.
• isEmail is still a loose format check, not full RFC 5322 validation.
• The review found no critical issues.