fix: resolve all failing tests across utility library

[stooit]

Summary

• calculator.ts: Throw Error("Division by zero") when divisor is 0 instead of returning Infinity
• string-utils.ts: Fix wordCount to handle multiple consecutive spaces; implement truncate with word-boundary logic
• task-manager.ts: Implement missing remove, update, and sortBy methods
• date-utils.ts: Use Math.round instead of Math.floor for relative day calculation (fixes off-by-one)
• validator.ts: Allow TLDs longer than 3 chars in email validation; allow port numbers in URL validation

Testing

All 60 tests pass across 5 test files. No test files were modified.

Assumptions

• truncate truncates at word boundaries with "..." appended, where total length (including "...") does not exceed maxLength
• sortBy("priority") sorts high > medium > low
• sortBy("createdAt") sorts oldest first
• Division by zero throws a generic Error (not a custom error class)