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 whitespace runs (/\s+/); implement truncate with word-boundary awareness
• task-manager.ts: Implement remove, update, and sortBy methods
• date-utils.ts: Use Math.round instead of Math.floor for day calculation in formatRelative
• validator.ts: Allow long TLDs (2+ chars) in isEmail; allow optional ports in isUrl

Assumptions

• formatRelative should use Math.round for day rounding (36h → 2 days)
• truncate truncates at word boundaries when possible, appending "..." which counts toward maxLength
• sortBy("priority") orders high > medium > low; sortBy("createdAt") oldest first
• No test files were modified; no new dependencies added

All 60 tests pass.