fix: resolve all failing tests across utility library

[stooit]

Summary

• calculator.ts: Throw error on division by zero instead of returning Infinity
• string-utils.ts: Fix wordCount to handle multiple consecutive spaces; implement truncate function with word-boundary truncation
• task-manager.ts: Implement remove, update, and sortBy methods
• date-utils.ts: Use Math.round for days calculation in formatRelative (fixes off-by-one at 36 hours)
• validator.ts: Allow long TLDs (e.g. .museum) in isEmail; allow port numbers in isUrl

Testing

All 60 tests pass, 0 failures. No test files were modified.

Assumptions

• formatRelative days calculation uses Math.round (test expects 36h → "2 days ago")
• truncate truncates at last word boundary within maxLength - 3, appends "..."
• sortBy("priority") orders high > medium > low
• sortBy("createdAt") orders oldest first