fix: resolve all 16 failing tests across 5 utility modules

[stooit]

Summary

• calculator.ts: Throw Error("Division by zero") when dividing by zero instead of returning Infinity
• string-utils.ts: Fix wordCount to split on /\s+/ (handles multiple consecutive spaces); implement truncate with word-boundary logic and ... suffix
• task-manager.ts: Implement remove(id), update(id, changes), and sortBy(field) methods (priority order: high > medium > low; createdAt: oldest first)
• date-utils.ts: Use Math.round instead of Math.floor for day calculation in formatRelative so 36 hours → 2 days
• validator.ts: Allow TLDs longer than 4 characters in email regex; accept port numbers in URL validation regex

Testing

All 60 tests pass (bun test). No previously passing tests were broken.

Assumptions

• truncate uses word-boundary truncation where the ... counts toward maxLength
• sortBy("priority") orders high > medium > low (descending severity)
• sortBy("createdAt") orders oldest first (ascending)
• No test files were modified; no new dependencies added