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 with word-boundary truncation
• date-utils.ts: Use Math.round for day calculation in formatRelative (fixes off-by-one at 36 hours)
• task-manager.ts: Implement remove, update, and sortBy methods
• validator.ts: Fix email regex to accept TLDs longer than 4 chars; fix URL regex to accept ports

Testing

All 60 tests pass (bun test). No test files were modified.

Assumptions

• truncate truncates at word boundaries when possible, falling back to hard truncation
• sortBy("priority") orders high > medium > low
• sortBy("createdAt") orders oldest first
• formatRelative uses Math.round for days (36h = 2 days ago)